fix(simulation): synthetic-overflow placements get the ocean-mask guard (T-1206)

synthetic_attractor now takes the terrain analysis and land-corrects
its pure-arithmetic position via a bounded nearest-land ring walk
(T-1116's pattern: row-major tie-break, column wrap, row clamp,
MAX_LAND_SEARCH_RING=128 sized empirically — real polar ocean bands
push nearest land up to 125 cells). Land positions pass through
UNTOUCHED — verified by direct before/after scan of all 267 real
bodies: 63 land-arithmetic placements byte-identical, and every
golden/determinism harness passes unchanged. The gap was real and
widespread: 46 of 109 synthetic-overflow placements sat in open water
at seed 42 (e.g. GJ903c at a genuine polar ocean cell); post-fix zero,
with all 109 preserved (confirmed at a second seed). Degradation is
defined and pinned: no land within the bound -> the synthetic
attractor is skipped and Phase 5's existing not-placed warning
reports it — never a panic, never a fabricated water position (the
bound never triggers on any scanned real body). 9 new unit tests;
road_graph's anchor comment and the D-210 amendment record the gap
CLOSED (validated).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/atlas/cascade.rs

@@ -196,6 +196,8 @@ impl CascadeSnapshot {
 ///
 /// `territorial_status` (D-212, from the body's `dominant_faction`) and `seed`
 /// drive the per-settlement spatial-character enrichment (#956, D-213/214/215).
+/// `terrain` (T-1206) gates the Phase-4 synthetic-overflow path against open
+/// water — see [`match_cities`]'s doc.
 fn run_layer3(
     attractors: &[GeographicAttractor],
     cities: &[CityRecord],
@@ -203,6 +205,7 @@ fn run_layer3(
     seed: SeedChain,
     grid_w: u32,
     grid_h: u32,
+    terrain: Option<&TerrainAnalysis>,
 ) -> Layer3Output {
     let matrix = CompatibilityMatrix::d195();
     let placements = match_cities(
@@ -214,6 +217,7 @@ fn run_layer3(
         grid_h,
         territorial_status,
         seed,
+        terrain,
     );
     Layer3Output { placements }
 }
@@ -321,8 +325,15 @@ pub fn run_cascade_from_heightmap(
             None => &[],
         };
         // cache seam: run_layer3 is a pure, deterministic function of
-        // (attractors, cities, territorial_status, seed) — wrap a persistent
-        // cache here when we add one (build-time bake or local cache; see #1021).
+        // (attractors, cities, territorial_status, seed, terrain) — wrap a
+        // persistent cache here when we add one (build-time bake or local
+        // cache; see #1021).
+        //
+        // T-1206: `snapshot.terrain_analysis` was just populated by the
+        // Topography block above (guaranteed `Some` here — Settlement >
+        // Topography in CascadeLayer's Ord, so the guard above always ran
+        // first) — passed by reference so the DistrictProfile/RoadGraph pass
+        // below still gets to consume (and drop) the same transient value.
         let l3 = run_layer3(
             attractors,
             cities,
@@ -330,6 +341,7 @@ pub fn run_cascade_from_heightmap(
             body_seed,
             snapshot.heightmap.width,
             snapshot.heightmap.height,
+            snapshot.terrain_analysis.as_ref(),
         );
         snapshot.layer3 = Some(l3);
     }