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>

tooling/economy-db/import_economics.py

@@ -297,16 +297,10 @@ DELETE FROM meta WHERE generator_name = 'generate_brands';
 -- (T6) would reject any committed DB that still carries the old row.
 DELETE FROM meta WHERE generator_name = 'generate_atlas';
 
--- Heightmap BLOB storage (D-202, #901)
-CREATE TABLE IF NOT EXISTS atlas_body_heightmaps (
-    body_id     TEXT PRIMARY KEY REFERENCES bodies(body_id) ON DELETE CASCADE,
-    width       INTEGER NOT NULL DEFAULT 512,
-    height      INTEGER NOT NULL DEFAULT 256,
-    data        BLOB NOT NULL,
-    sea_level   REAL NOT NULL DEFAULT 0.0,
-    imported_at TEXT NOT NULL DEFAULT (datetime('now'))
-);
-CREATE INDEX IF NOT EXISTS idx_atlas_body_heightmaps_body ON atlas_body_heightmaps(body_id);
+-- Drop the retired heightmap BLOB table (D-202 amended, #963): canonical
+-- elevation is now a per-body 16-bit grayscale heightmap.png file, not a DB
+-- BLOB. The Rust loader reads the PNG; nothing reads this table anymore.
+DROP TABLE IF EXISTS atlas_body_heightmaps;
 
 -- City name reservations (D-207, #902)
 CREATE TABLE IF NOT EXISTS atlas_city_names (