fix(simulation): address PR #160 review (climate + morphology)

Hoshe REQUEST_CHANGES + Tyre APPROVE:

- Riparian vegetation logic was doubly-wrong: it granted the micro-oasis only
  in the impossible cold case (temp < -50, no liquid water) and DENIED it in the
  intended hyper-arid case. Split: extreme cold -> Barren always; hyper-arid ->
  RiparianScrub iff near perennial water. (Hoshe #2)
- morphology_zone_has_exactly_17_variants was misleading (asserted >=16). Rename
  to morphology_zone_region_scale_emits_16_of_17, tighten to ==16, and document
  that BraidedPlain is the lone region-unreachable zone (needs lithology, deferred
  to ChunkContext) and that Lake IS reachable. Tyre's 'Lake also deferred' was a
  false positive (ocean 60-79 -> Lake). (Hoshe #1, Tyre #1)
- derive_morphology_zone docstring now lists the real multi-path emission sites
  for TidalFlat/Wetland, ValleyFloor/RiverBank as their own gates, and BraidedPlain's
  deferral — the §6 parent-family names are descriptive, not exhaustive. (Hoshe #3, Tyre #2/#3/#4)
- D-239 record: added a T-1025/T-1027 implementation note (enum freeze location,
  16/17 region reachability + BraidedPlain deferral, §7 gate-ordering interpretation).

No vocabulary change. cargo test passes, clippy -D warnings clean, fmt clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/atlas/region_profile.rs

@@ -366,11 +366,16 @@ pub fn derive_vegetation(
     };
     let temp_i = temp_c as i32;
 
-    // Hyper-arid or extreme cold → Barren regardless of elevation.
-    // These are the absolute limits; no vegetation survives.
-    if moisture_q < 5 || temp_i < -50 {
-        return if near_perennial_water && moisture_q >= 5 {
-            VegetationClass::RiparianScrub // water creates a micro-oasis even in arid extremes
+    // Extreme cold → Barren regardless of water proximity: below ~−50 °C mean
+    // there is no liquid surface water, so a perennial-waterway micro-oasis is
+    // physically impossible (surface ice is geology per D-227, not vegetation).
+    if temp_i < -50 {
+        return VegetationClass::Barren;
+    }
+    // Hyper-arid → Barren, but a perennial waterway sustains a riparian micro-oasis.
+    if moisture_q < 5 {
+        return if near_perennial_water {
+            VegetationClass::RiparianScrub
         } else {
             VegetationClass::Barren
         };
@@ -479,11 +484,17 @@ pub fn derive_river_threshold(tectonic: TectonicClass, precip: PrecipitationClas
 ///   8. MeanderReach — gate: gentle slope + some water presence
 ///   9. AlluvialPlain — fallback (§5)
 ///
-/// Sub-classifications (§6): after family selection, apply:
-///   - Alpine: IncisedGorge family + elev_q ≥ ALPINE_ELEV_THRESHOLD
-///   - Wetland: AlluvialPlain/Delta family + slope_q ≤ 5 + moisture ≥ 60 (§8 Wetland ≤5°)
-///   - TidalFlat: AlluvialPlain/CliffCoast family + ocean_fraction_q ≥ TIDAL_OCEAN_MIN + very low elev
-///   - Estuarine: Delta family + ocean signal (brackish tidal zone at river mouth)
+/// Sub-classifications (§6): after family selection, apply. NOTE: the §6 "parent
+/// family" names are descriptive of the typical source, not exhaustive — these
+/// zones derive from morphological conditions (low elev + tidal signal; flat +
+/// moist) that legitimately overlap several families, so a zone can arise from
+/// more than one family context. The actual emission sites:
+///   - Alpine: IncisedGorge family + elev_q ≥ ALPINE_ELEV_THRESHOLD.
+///   - Wetland: flat (slope_q ≤ 5) + moisture ≥ 60; emitted from the AlluvialPlain fallback AND within the MeanderReach gate (§8 Wetland ≤5° flats).
+///   - TidalFlat: very low elev + ocean signal; emitted from the BraidedDelta and DuneStrand gates AND a standalone low-coast gate after Family 5.
+///   - Estuarine: Delta family + strong ocean signal (brackish tidal zone, river mouth).
+///   - ValleyFloor and RiverBank are their own gates (ValleyFloor between IncisedGorge and MeanderReach; RiverBank within the MeanderReach gate).
+///   - BraidedPlain (§6) is NOT emitted at region scale — distinguishing it from Delta needs a lithology signal (§8 Gravel→braided) RegionProfile lacks; deferred to ChunkContext (see the D-239 §6 implementation note).
 ///
 /// D-010: all gates are integer comparisons. No float arithmetic in this function.
 fn derive_morphology_zone(
@@ -1609,9 +1620,10 @@ mod tests {
     }
 
     #[test]
-    fn morphology_zone_has_exactly_17_variants() {
-        // Enumerate all reachable zones through the classifier across a
-        // representative grid of inputs to verify all 17 are reachable.
+    fn morphology_zone_region_scale_emits_16_of_17() {
+        // Enumerate the zones the region-scale classifier can emit across a
+        // representative input grid. 16 of the 17 frozen zones are reachable here;
+        // BraidedPlain is the lone exception — see the assertion comment below.
         use std::collections::BTreeSet;
         let mut seen: BTreeSet<u8> = BTreeSet::new();
 
@@ -1636,13 +1648,15 @@ mod tests {
                 }
             }
         }
-        // BraidedPlain is not yet emitted by classifier (it's a future ChunkContext
-        // sub-classification at finer scale); all others must be reachable.
-        // Assert at least 16 of 17 zones are reachable via the region-scale classifier.
-        assert!(
-            seen.len() >= 16,
-            "only {}/17 zones reachable by classifier: {seen:?}",
-            seen.len()
+        // BraidedPlain (discriminant 11) is the ONE frozen zone the region-scale
+        // classifier never emits: distinguishing it from Delta needs a lithology
+        // signal (§8 Gravel→braided) that RegionProfile doesn't carry yet, so it is
+        // deferred to ChunkContext (D-239 §6 implementation note). All other 16 are
+        // reachable. (Lake IS reachable — ocean_fraction 60–79 → Lake, ≥80 → OpenOcean.)
+        assert_eq!(
+            seen.len(),
+            16,
+            "expected exactly 16/17 region-reachable zones (BraidedPlain deferred), got: {seen:?}"
         );
     }