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>

governance/decisions/architecture.md

@@ -1771,6 +1771,7 @@ Technical foundation decisions that constrain implementation: engine, client-ser
 - **Raised by:** tile-derivation-contract workshop (Tyre — refinement chain, warp, determinism; Gestalt — 8 families, game-feel; Troblum — feasibility, warp precision, scale corrections; Miri — believability laws, vocabulary, lore reconciliation), lead-interviewed decisions + an adversarial verification pass, 2026-06-07.
 - **Dissent:** Tyre's initial cross-family elevation-blend was resolved against (prevent-at-source). Early-integer-truncation of the warp (raised against Gestalt's `ElevationDelta` ranges and by Tyre) was resolved against in favour of f64-to-voxel.
 - **Implementation note (T-1024, 2026-06-07):** §1 per-body `RIVER_THRESHOLD` is now a derived field (`derive_river_threshold`) on `RegionProfile`. §2 district temperature is a nullable `f32` on `RegionProfile` sampled from a per-region climate derivation (`derive_temperature_c`); moisture is a separate integer primitive (`derive_moisture_q`, 0–100). Both populate the `RegionProfile` carrier during the L4 cascade run. Climate inputs use a **hybrid strategy**: stellar luminosity and spectral class are sourced from the import pipeline (`bodies.axial_tilt_deg`, `star_systems.spectral_class`); greenhouse offsets and diurnal amplitudes are tunable at runtime via `server/data/climate_constants.toml` (source-canonical TOML, not hardcoded).
+- **Implementation note (T-1025/T-1027, 2026-06-08):** §6's frozen 17-zone `MorphologyZone` enum is realised in `server/src/simulation/generator.rs` (`repr(u8)`, discriminant-pinned). The §5 8-family gated classifier + §7 compatibility-matrix invariants live in `derive_morphology_zone` (`region_profile.rs`). **16 of the 17 zones are reachable at RegionProfile scale; `BraidedPlain` is the exception** — distinguishing it from `Delta` needs a lithology signal (§8 Gravel→braided) that `RegionProfile` does not carry, so `BraidedPlain` is **deferred to ChunkContext** sub-classification. The §7 compatibility invariant is enforced as a **classifier-gate-ordering** property (a build-time test that a single region cannot yield a forbidden pair), per §7's "build-time test" language; genuine cross-region sharp seams (cliff↔fjord, lithology faults) remain permitted. §2 climate-derived fields (`precipitation_class`, `glaciation_grade`, `vegetation_class`) all derive from the temperature(+moisture) primitive (T-1025).
 - **Cross-reference:** [D-227](#d-227) (derive-don't-store voxel model), [D-228](#d-228) (composite tile axes / cohesion / seasonal state), [D-210](#d-210) (temperature proxy — formalised), [D-203](#d-203) (BodyWorldState cache), [D-206](#d-206) (background analysis pass), [D-208](#d-208) (drainage / D8), [D-010](#d-010) (determinism), [D-234](#d-234) (street/footprint geometry — consumes morphology), [D-142](content.md#d-142) (zone types), [D-217](#d-217) (tile condition), [Q-102](../questions/architecture.md#q-102) (cohesion = the warp), [Q-103](../questions/architecture.md#q-103) (mutator schema — open), [Q-105](../questions/architecture.md#q-105) (seasonal/clock state — temperature/ElevationDelta forward contract)
 
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