From 07f0fa307fa1e2594edf84bcde4865f7009288fe Mon Sep 17 00:00:00 2001
From: Jeroen Schweitzer <jpmschweitzer@gmail.com>
Date: Sat, 25 Jul 2026 13:08:26 +0200
Subject: [PATCH] =?UTF-8?q?fix(simulation):=20region=20baseline=20latitude?=
 =?UTF-8?q?=20=E2=80=94=20equator-anchored=20signed=20rows=20(T-1186)?=
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region_centre_latitude_deg mapped row 0 to the north pole with lat_frac
clamped [0,1] while the derive core floor-divides equator-anchored signed
world metres — every northern-hemisphere region clamped to +90 (polar
everywhere) and the southern hemisphere read as compressed northern
tropics. Now mirrors the derive core's exact inverse mapping (lat_frac =
centre_y/meridian clamped [-0.5,0.5], lat = -frac*180). The ticket's
in-the-wild datapoint flips as predicted: GJ338Bd region (136,43), old
baseline +13.94N/26.8C -> new -76.06S/10.3C; the 76S district cell now
derives Lake/2.07C/Light glaciation. Goldens regenerated (believability +
window derivation, stable on rerun); two region_profile unit tests
repositioned — their probe coordinates meant different places under the
old convention. step_canvas rung-0 already used the correct signed
convention; layer_proxy's pole-anchored pseudo-grid stays deferred to
T-1181's rung-0 rebuild per D-256(f).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 server/src/atlas/region_profile.rs            |  68 +++++--
 server/tests/golden/believability.json        |  18 +-
 .../golden/window_derivation_golden.json      | 174 +++++++++---------
 3 files changed, 150 insertions(+), 110 deletions(-)

diff --git a/server/src/atlas/region_profile.rs b/server/src/atlas/region_profile.rs
index b05e12317..ee5176892 100644
--- a/server/src/atlas/region_profile.rs
+++ b/server/src/atlas/region_profile.rs
@@ -258,11 +258,32 @@ pub fn derive_region_baseline_c(
 
 /// Compute the latitude of a region centre from its grid position.
 ///
-/// The region grid is equirectangular. Row 0 sits at the north pole; the
-/// equator is in the middle. Returns degrees: +90.0 = north pole, −90.0 =
-/// south pole.
+/// **Equator-anchored SIGNED row convention (T-1186 fix)** — this MUST mirror
+/// the canonical derive core's own inverse mapping
+/// (`derive_at_metres_with_riparian`, district_profile.rs: `lat_frac = (wy /
+/// meridian_m).clamp(-0.5, 0.5)`, `lat_deg = -lat_frac * 180.0`, keyed on
+/// equator-anchored signed world metres via `district_to_region`'s
+/// `div_euclid`). `region_pos.1` is therefore SIGNED: negative rows are
+/// north, positive rows are south, row 0 straddles the equator — NOT the
+/// pole-anchored non-negative convention this function used before T-1186
+/// (row 0 = north pole, `lat_frac` clamped to `[0, 1]`), which silently
+/// disagreed with the derive core it feeds (D-256(a): one absolute-metre
+/// derive core, one inverse mapping — this was the second, wrong, mapping).
+///
+/// Returns degrees: +90.0 = north pole, −90.0 = south pole.
 ///
 /// If no body radius is available (tiny test bodies), the region lat is 0.0.
+///
+/// **Convention note (T-1186 audit):** two OTHER call sites in this codebase
+/// still use the old non-negative pole-anchored row convention and do NOT
+/// call this function — `step_canvas::build_step_canvas`'s Global (rung-0)
+/// canvas (deliberately switched to signed-equator rows ahead of this fix,
+/// see its doc comment) and `layer_proxy::build_region_grid` /
+/// `cascade::LayerRegionOutput` (still pole-anchored, non-negative `ry`,
+/// D-256(f)-deferred to T-1181's rung-0 rebuild — its sole reader is the
+/// stale body-view overlay, no consumer disagreement in this ticket's blast
+/// radius). See T-1186's report for the proposed one-convention unification;
+/// this function's fix does not by itself touch either of those call sites.
 pub fn region_centre_latitude_deg(region_pos: RegionPos, body_radius_km: Option<f64>) -> f64 {
     let Some(r_km) = body_radius_km else {
         return 0.0;
@@ -270,13 +291,17 @@ pub fn region_centre_latitude_deg(region_pos: RegionPos, body_radius_km: Option<
     if r_km <= 0.0 {
         return 0.0;
     }
-    // The meridian spans πR km. Each region is REGION_M metres tall.
-    // region_y = 0 maps to the north pole (lat +90), rising y → south.
+    // The meridian spans πR km. Each region is REGION_M metres tall,
+    // equator-anchored: region row 0 straddles the equator, negative rows
+    // are north (mirrors district_to_region's div_euclid on signed,
+    // equator-anchored district positions).
     let meridian_m = std::f64::consts::PI * r_km * 1_000.0;
     let region_centre_y_m = (region_pos.1 as f64 + 0.5) * scale::REGION_M as f64;
-    // Clamp: lat_frac in [0, 1]; 0 = N pole (+90°), 1 = S pole (−90°).
-    let lat_frac = (region_centre_y_m / meridian_m).clamp(0.0, 1.0);
-    90.0 - lat_frac * 180.0
+    // Clamp: lat_frac in [-0.5, 0.5]; -0.5 = N pole, +0.5 = S pole — the exact
+    // inverse of derive_at_metres_with_riparian's `lat_frac = (wy /
+    // meridian_m).clamp(-0.5, 0.5)`.
+    let lat_frac = (region_centre_y_m / meridian_m).clamp(-0.5, 0.5);
+    -lat_frac * 180.0
 }
 
 /// Build a [`RegionProfile`] for the region at `region_pos`.
@@ -611,12 +636,16 @@ mod tests {
 
     #[test]
     fn equatorial_region_is_warmer_than_polar() {
-        // With a body radius, north-pole region vs equatorial region.
+        // T-1186: region rows (RegionPos.1, the Y/row component — RegionPos is
+        // (x, y)) are equator-anchored SIGNED (row 0 straddles the equator;
+        // negative = north, matching the derive core's floor-divide of
+        // equator-anchored world metres). Region (0, 0) is therefore near the
+        // EQUATOR, not the pole — (0, -97) is the genuinely polar region on
+        // this Earth-radius test body (~97.7 regions pole-to-pole).
         let params = earth_params();
         let constants = ClimateConstants::default();
-        // Region (0, 0) is near the north pole; (97, 15) is roughly equatorial.
-        let polar = build_region_profile(body_seed(), &params, &constants, (0, 0));
-        let equatorial = build_region_profile(body_seed(), &params, &constants, (97, 15));
+        let polar = build_region_profile(body_seed(), &params, &constants, (0, -97));
+        let equatorial = build_region_profile(body_seed(), &params, &constants, (0, 0));
         match (polar.clock.mean_temp_c, equatorial.clock.mean_temp_c) {
             (Some(p), Some(e)) => assert!(
                 p < e,
@@ -746,12 +775,23 @@ mod tests {
 
     #[test]
     fn edge_fuzz_varies_with_body_id() {
+        // T-1186: (50, 50) sat right at the equator under the new
+        // equator-anchored-signed row convention (region row 0), where
+        // earth_params()'s ocean/breathable/temperate band saturates at the
+        // 28°C warm ceiling for every neighbour regardless of warp — a
+        // fix-induced false negative, not a real regression (the OLD
+        // pole-anchored convention happened to place row 0 near the pole,
+        // where the band isn't saturated). (50, 2400) — a mid-latitude
+        // district — also turned out to be a coincidental warp collision at
+        // this seed (both bodies landing on the same blend). (73, 2455) is a
+        // confirmed non-degenerate probe: comfortably inside the unsaturated
+        // band with the two bodies' warps genuinely diverging.
         let params = earth_params();
         let constants = ClimateConstants::default();
         let a = region_baseline_at_district(
             42,
             "BodyA",
-            (50, 50),
+            (73, 2455),
             &params,
             &constants,
             body_seed(),
@@ -760,7 +800,7 @@ mod tests {
         let b = region_baseline_at_district(
             42,
             "BodyB",
-            (50, 50),
+            (73, 2455),
             &params,
             &constants,
             body_seed(),
diff --git a/server/tests/golden/believability.json b/server/tests/golden/believability.json
index 2711991a6..571779224 100644
--- a/server/tests/golden/believability.json
+++ b/server/tests/golden/believability.json
@@ -8,7 +8,7 @@
       "moisture_q": {
         "min": 17,
         "max": 97,
-        "distinct": 74
+        "distinct": 75
       },
       "elev_q": {
         "min": 0,
@@ -17,27 +17,27 @@
       },
       "slope_q": {
         "min": 0,
-        "max": 31,
-        "distinct": 31
+        "max": 27,
+        "distinct": 28
       },
       "ocean_fraction_q": {
         "min": 0,
         "max": 100,
-        "distinct": 44
+        "distinct": 47
       },
       "morphology_zones": 9,
       "vegetation_classes": 4,
-      "terrain_materials": 4,
+      "terrain_materials": 3,
       "voxel_relief_m": 28,
-      "micro_habitat_distinct": 2
+      "micro_habitat_distinct": 3
     },
     "coherence": {
       "water_districts": 46,
-      "water_districts_wet": 46,
+      "water_districts_wet": 45,
       "drainage_samples": 0,
       "drainage_monotonic": 0,
       "vegetation_samples": 64,
-      "vegetated_districts": 8
+      "vegetated_districts": 7
     }
   },
   {
@@ -67,7 +67,7 @@
         "distinct": 36
       },
       "morphology_zones": 6,
-      "vegetation_classes": 2,
+      "vegetation_classes": 3,
       "terrain_materials": 2,
       "voxel_relief_m": 22,
       "micro_habitat_distinct": 0
diff --git a/server/tests/golden/window_derivation_golden.json b/server/tests/golden/window_derivation_golden.json
index 2470c00fb..fa1678e81 100644
--- a/server/tests/golden/window_derivation_golden.json
+++ b/server/tests/golden/window_derivation_golden.json
@@ -7,14 +7,14 @@
     "min_wl_m": 4096,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
-    "precipitation": 2,
-    "slope_q": 3,
+    "glaciation": 0,
+    "precipitation": 3,
+    "slope_q": 2,
     "elev_q": 27,
     "ocean_fraction_q": 0,
-    "temperature_dc": -48,
+    "temperature_dc": 118,
     "moisture_q": 59,
-    "vegetation": 2
+    "vegetation": 3
   },
   {
     "label": "coastal_a",
@@ -24,14 +24,14 @@
     "min_wl_m": 1024,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 2,
-    "precipitation": 2,
-    "slope_q": 4,
-    "elev_q": 32,
+    "glaciation": 0,
+    "precipitation": 3,
+    "slope_q": 3,
+    "elev_q": 31,
     "ocean_fraction_q": 0,
-    "temperature_dc": -74,
+    "temperature_dc": 97,
     "moisture_q": 59,
-    "vegetation": 2
+    "vegetation": 3
   },
   {
     "label": "coastal_a",
@@ -41,14 +41,14 @@
     "min_wl_m": 0,
     "morphology": 16,
     "tectonic": 0,
-    "glaciation": 1,
-    "precipitation": 2,
+    "glaciation": 0,
+    "precipitation": 3,
     "slope_q": 0,
     "elev_q": 26,
     "ocean_fraction_q": 0,
-    "temperature_dc": -43,
+    "temperature_dc": 123,
     "moisture_q": 60,
-    "vegetation": 2
+    "vegetation": 3
   },
   {
     "label": "coastal_b",
@@ -58,14 +58,14 @@
     "min_wl_m": 4096,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 2,
-    "precipitation": 1,
+    "glaciation": 0,
+    "precipitation": 2,
     "slope_q": 3,
-    "elev_q": 30,
+    "elev_q": 29,
     "ocean_fraction_q": 0,
-    "temperature_dc": -68,
+    "temperature_dc": 112,
     "moisture_q": 55,
-    "vegetation": 2
+    "vegetation": 3
   },
   {
     "label": "coastal_b",
@@ -75,14 +75,14 @@
     "min_wl_m": 1024,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
-    "precipitation": 2,
-    "slope_q": 4,
-    "elev_q": 26,
+    "glaciation": 0,
+    "precipitation": 3,
+    "slope_q": 3,
+    "elev_q": 25,
     "ocean_fraction_q": 0,
-    "temperature_dc": -47,
+    "temperature_dc": 132,
     "moisture_q": 56,
-    "vegetation": 2
+    "vegetation": 3
   },
   {
     "label": "coastal_b",
@@ -92,14 +92,14 @@
     "min_wl_m": 0,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
-    "precipitation": 2,
+    "glaciation": 0,
+    "precipitation": 3,
     "slope_q": 0,
     "elev_q": 26,
     "ocean_fraction_q": 0,
-    "temperature_dc": -47,
+    "temperature_dc": 127,
     "moisture_q": 57,
-    "vegetation": 2
+    "vegetation": 3
   },
   {
     "label": "coastal_c",
@@ -109,14 +109,14 @@
     "min_wl_m": 4096,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
-    "precipitation": 2,
+    "glaciation": 0,
+    "precipitation": 3,
     "slope_q": 3,
     "elev_q": 24,
     "ocean_fraction_q": 0,
-    "temperature_dc": -30,
+    "temperature_dc": 131,
     "moisture_q": 59,
-    "vegetation": 2
+    "vegetation": 3
   },
   {
     "label": "coastal_c",
@@ -126,14 +126,14 @@
     "min_wl_m": 1024,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 2,
-    "precipitation": 2,
-    "slope_q": 4,
-    "elev_q": 28,
+    "glaciation": 0,
+    "precipitation": 3,
+    "slope_q": 3,
+    "elev_q": 27,
     "ocean_fraction_q": 0,
-    "temperature_dc": -51,
+    "temperature_dc": 115,
     "moisture_q": 58,
-    "vegetation": 2
+    "vegetation": 3
   },
   {
     "label": "coastal_c",
@@ -143,14 +143,14 @@
     "min_wl_m": 0,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
-    "precipitation": 2,
+    "glaciation": 0,
+    "precipitation": 3,
     "slope_q": 0,
     "elev_q": 27,
     "ocean_fraction_q": 0,
-    "temperature_dc": -45,
+    "temperature_dc": 115,
     "moisture_q": 58,
-    "vegetation": 2
+    "vegetation": 3
   },
   {
     "label": "inland",
@@ -160,12 +160,12 @@
     "min_wl_m": 4096,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
+    "glaciation": 0,
     "precipitation": 2,
-    "slope_q": 4,
+    "slope_q": 3,
     "elev_q": 25,
     "ocean_fraction_q": 0,
-    "temperature_dc": 0,
+    "temperature_dc": 90,
     "moisture_q": 52,
     "vegetation": 3
   },
@@ -177,12 +177,12 @@
     "min_wl_m": 1024,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
+    "glaciation": 0,
     "precipitation": 2,
-    "slope_q": 5,
+    "slope_q": 4,
     "elev_q": 20,
     "ocean_fraction_q": 0,
-    "temperature_dc": 26,
+    "temperature_dc": 116,
     "moisture_q": 52,
     "vegetation": 3
   },
@@ -194,12 +194,12 @@
     "min_wl_m": 0,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
+    "glaciation": 0,
     "precipitation": 2,
     "slope_q": 0,
     "elev_q": 22,
     "ocean_fraction_q": 0,
-    "temperature_dc": 16,
+    "temperature_dc": 106,
     "moisture_q": 52,
     "vegetation": 3
   },
@@ -216,7 +216,7 @@
     "slope_q": 3,
     "elev_q": 63,
     "ocean_fraction_q": 0,
-    "temperature_dc": -69,
+    "temperature_dc": -120,
     "moisture_q": 43,
     "vegetation": 1
   },
@@ -233,7 +233,7 @@
     "slope_q": 4,
     "elev_q": 61,
     "ocean_fraction_q": 0,
-    "temperature_dc": -59,
+    "temperature_dc": -120,
     "moisture_q": 43,
     "vegetation": 1
   },
@@ -250,7 +250,7 @@
     "slope_q": 0,
     "elev_q": 62,
     "ocean_fraction_q": 0,
-    "temperature_dc": -64,
+    "temperature_dc": -120,
     "moisture_q": 43,
     "vegetation": 1
   },
@@ -262,12 +262,12 @@
     "min_wl_m": 4096,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
+    "glaciation": 0,
     "precipitation": 2,
     "slope_q": 2,
     "elev_q": 8,
     "ocean_fraction_q": 0,
-    "temperature_dc": 14,
+    "temperature_dc": 91,
     "moisture_q": 49,
     "vegetation": 3
   },
@@ -279,12 +279,12 @@
     "min_wl_m": 1024,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
+    "glaciation": 0,
     "precipitation": 2,
-    "slope_q": 3,
-    "elev_q": 4,
+    "slope_q": 2,
+    "elev_q": 5,
     "ocean_fraction_q": 0,
-    "temperature_dc": 34,
+    "temperature_dc": 106,
     "moisture_q": 50,
     "vegetation": 3
   },
@@ -296,12 +296,12 @@
     "min_wl_m": 0,
     "morphology": 8,
     "tectonic": 0,
-    "glaciation": 1,
+    "glaciation": 0,
     "precipitation": 2,
     "slope_q": 0,
     "elev_q": 11,
     "ocean_fraction_q": 0,
-    "temperature_dc": -2,
+    "temperature_dc": 75,
     "moisture_q": 49,
     "vegetation": 3
   },
@@ -624,7 +624,7 @@
     "slope_q": 3,
     "elev_q": 25,
     "ocean_fraction_q": 0,
-    "temperature_dc": 497,
+    "temperature_dc": 749,
     "moisture_q": 60,
     "vegetation": 3
   },
@@ -641,7 +641,7 @@
     "slope_q": 6,
     "elev_q": 13,
     "ocean_fraction_q": 0,
-    "temperature_dc": 560,
+    "temperature_dc": 811,
     "moisture_q": 62,
     "vegetation": 3
   },
@@ -658,7 +658,7 @@
     "slope_q": 0,
     "elev_q": 28,
     "ocean_fraction_q": 0,
-    "temperature_dc": 482,
+    "temperature_dc": 733,
     "moisture_q": 59,
     "vegetation": 3
   },
@@ -675,7 +675,7 @@
     "slope_q": 4,
     "elev_q": 34,
     "ocean_fraction_q": 0,
-    "temperature_dc": 456,
+    "temperature_dc": 697,
     "moisture_q": 58,
     "vegetation": 3
   },
@@ -692,7 +692,7 @@
     "slope_q": 6,
     "elev_q": 24,
     "ocean_fraction_q": 0,
-    "temperature_dc": 508,
+    "temperature_dc": 749,
     "moisture_q": 60,
     "vegetation": 3
   },
@@ -709,7 +709,7 @@
     "slope_q": 0,
     "elev_q": 28,
     "ocean_fraction_q": 0,
-    "temperature_dc": 487,
+    "temperature_dc": 728,
     "moisture_q": 59,
     "vegetation": 3
   },
@@ -726,7 +726,7 @@
     "slope_q": 2,
     "elev_q": 28,
     "ocean_fraction_q": 0,
-    "temperature_dc": 486,
+    "temperature_dc": 729,
     "moisture_q": 59,
     "vegetation": 3
   },
@@ -743,7 +743,7 @@
     "slope_q": 4,
     "elev_q": 33,
     "ocean_fraction_q": 0,
-    "temperature_dc": 460,
+    "temperature_dc": 703,
     "moisture_q": 59,
     "vegetation": 3
   },
@@ -760,7 +760,7 @@
     "slope_q": 0,
     "elev_q": 29,
     "ocean_fraction_q": 0,
-    "temperature_dc": 481,
+    "temperature_dc": 724,
     "moisture_q": 59,
     "vegetation": 3
   },
@@ -777,7 +777,7 @@
     "slope_q": 3,
     "elev_q": 24,
     "ocean_fraction_q": 0,
-    "temperature_dc": 565,
+    "temperature_dc": 691,
     "moisture_q": 49,
     "vegetation": 3
   },
@@ -794,7 +794,7 @@
     "slope_q": 4,
     "elev_q": 21,
     "ocean_fraction_q": 0,
-    "temperature_dc": 581,
+    "temperature_dc": 707,
     "moisture_q": 49,
     "vegetation": 3
   },
@@ -811,7 +811,7 @@
     "slope_q": 0,
     "elev_q": 23,
     "ocean_fraction_q": 0,
-    "temperature_dc": 571,
+    "temperature_dc": 697,
     "moisture_q": 49,
     "vegetation": 3
   },
@@ -828,7 +828,7 @@
     "slope_q": 3,
     "elev_q": 68,
     "ocean_fraction_q": 0,
-    "temperature_dc": 545,
+    "temperature_dc": 300,
     "moisture_q": 29,
     "vegetation": 2
   },
@@ -845,7 +845,7 @@
     "slope_q": 4,
     "elev_q": 65,
     "ocean_fraction_q": 0,
-    "temperature_dc": 561,
+    "temperature_dc": 300,
     "moisture_q": 30,
     "vegetation": 2
   },
@@ -862,7 +862,7 @@
     "slope_q": 0,
     "elev_q": 66,
     "ocean_fraction_q": 0,
-    "temperature_dc": 555,
+    "temperature_dc": 300,
     "moisture_q": 30,
     "vegetation": 2
   },
@@ -879,7 +879,7 @@
     "slope_q": 4,
     "elev_q": 14,
     "ocean_fraction_q": 0,
-    "temperature_dc": 500,
+    "temperature_dc": 597,
     "moisture_q": 47,
     "vegetation": 3
   },
@@ -896,7 +896,7 @@
     "slope_q": 6,
     "elev_q": 5,
     "ocean_fraction_q": 0,
-    "temperature_dc": 547,
+    "temperature_dc": 644,
     "moisture_q": 49,
     "vegetation": 3
   },
@@ -913,7 +913,7 @@
     "slope_q": 0,
     "elev_q": 18,
     "ocean_fraction_q": 0,
-    "temperature_dc": 480,
+    "temperature_dc": 576,
     "moisture_q": 47,
     "vegetation": 3
   },
@@ -930,7 +930,7 @@
     "slope_q": 4,
     "elev_q": 0,
     "ocean_fraction_q": 0,
-    "temperature_dc": 62,
+    "temperature_dc": 280,
     "moisture_q": 56,
     "vegetation": 6
   },
@@ -947,7 +947,7 @@
     "slope_q": 5,
     "elev_q": 0,
     "ocean_fraction_q": 0,
-    "temperature_dc": 62,
+    "temperature_dc": 280,
     "moisture_q": 56,
     "vegetation": 6
   },
@@ -964,7 +964,7 @@
     "slope_q": 0,
     "elev_q": 0,
     "ocean_fraction_q": 0,
-    "temperature_dc": 62,
+    "temperature_dc": 280,
     "moisture_q": 56,
     "vegetation": 6
   },
@@ -981,7 +981,7 @@
     "slope_q": 0,
     "elev_q": 71,
     "ocean_fraction_q": 0,
-    "temperature_dc": -120,
+    "temperature_dc": -89,
     "moisture_q": 55,
     "vegetation": 1
   },
@@ -998,7 +998,7 @@
     "slope_q": 2,
     "elev_q": 75,
     "ocean_fraction_q": 0,
-    "temperature_dc": -120,
+    "temperature_dc": -110,
     "moisture_q": 54,
     "vegetation": 1
   },
@@ -1015,7 +1015,7 @@
     "slope_q": 0,
     "elev_q": 70,
     "ocean_fraction_q": 0,
-    "temperature_dc": -120,
+    "temperature_dc": -84,
     "moisture_q": 54,
     "vegetation": 1
   }