feat: complete file-based heightmap migration — tEXt sea_level, client relief, drop BLOB (#963)

- heightmap.rs: read sea_level from the PNG tEXt chunk (bake writes it),
  default-fallback param; new test reads_sea_level_from_text_chunk.
- client atlas_viewer.gd: load reliefmap.png (color display) instead of
  heightmap.png (now 16-bit grayscale elevation, cascade-only).
- drop atlas_body_heightmaps: removed from systems-schema.sql; DROP TABLE in
  import_economics MIGRATION_SQL (the PNG is the store now).
- D-202 amendment: implementation-status note (consumer + producer done),
  resolving the review's 'reads done but producer pending' point.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

decisions/architecture.md

@@ -927,6 +927,7 @@ Technical foundation decisions that constrain implementation: engine, client-ser
   - **Multi-resolution:** the stored heightmap is high-res for the lower layers (region/block/tile sample local detail); **Layer 1** (continental drainage/basins/mountain-ranges) calls `BodyHeightmap::downsample` to the `GRID_W×GRID_H = 512×256` working resolution first, decoupling continental compute cost (~45ms) from stored resolution.
   - **Rust loader:** `heightmap.rs::load_heightmap_png` reads the 16-bit grayscale PNG (via the `png` crate), normalizes to f32 [0,1]; rejects RGB (a reliefmap can't be misread as elevation). `sea_level` becomes body metadata carried alongside (not in the PNG).
   - **`atlas_body_heightmaps` is dropped**; `import_heightmaps.py` writes the PNG file instead of a DB row. The bake runs in the content pipeline (numpy/scipy) once; the runtime cascade is pure Rust loading the file.
+  - **Implementation status (#963):** Consumer done — `heightmap.rs::load_heightmap_png` reads the 16-bit grayscale PNG + `sea_level` tEXt chunk, rejects RGB, downsamples for Layer 1. Producer done — `import_heightmaps.py` is the bake (rename legacy `heightmap.png`→`reliefmap.png` for all bodies incl. Sol; for non-Sol inhabited bodies write a fresh clean `reliefmap.png` + 16-bit `heightmap.png` from `simulate()` at the bumped 1024×512 grid). `atlas_body_heightmaps` dropped via MIGRATION_SQL + removed from `systems-schema.sql`. Godot client (`atlas_viewer.gd`) loads `reliefmap.png` for display. Sim determinism guarded by `test_sim_determinism.py`.
 - **Rationale:** Storing heightmaps in `systems.db` keeps the DB as the single source of truth for all generation inputs, avoids a separate file-fetching path in the Rust server, and allows the pre-push hook (asset pipeline rules) to detect stale heightmap data. The float32 LE layout matches what NumPy and PIL produce natively, minimizing conversion overhead in the Python pipeline. *(Superseded by the #963 amendment above — the file-based model won out because the DB-as-single-source goal conflicts with binary-merge-conflict avoidance and DB size; a per-body committed PNG is itself a queryable, diffable-by-render asset.)*
 - **Ticket:** #901 (schema), #906 (import), #916 (Rust loader)
 - **Raised by:** Generation cascade workshop (#897)