fix(tooling): address PR #129 review — atlas generator, schema, Makefile

- ON DELETE CASCADE added to every atlas_* foreign key (atlas_body_grids,
  atlas_cities, atlas_roads, atlas_railroads, atlas_pois, atlas_rivers,
  atlas_oceans, atlas_mountain_ranges). Previously, deleting a body from
  the bodies table or NULL-ing its terrain_reference would leave orphan
  atlas rows forever — sync_markers_to_db only cleans up for bodies it
  re-processes. The existing atlas tables in systems.db were dropped and
  recreated with the new constraint; FK list now reports CASCADE.

- Atlas DDL deduplicated. systems-schema.sql is now the single source of
  truth, bracketed by `-- BEGIN ATLAS INDEX` / `-- END ATLAS INDEX`
  markers. generate_atlas.py reads that block via `_load_atlas_schema()`
  and applies it at runtime, so there is no second copy of the DDL to
  keep in sync. Adding a column requires one edit, not two.

- Uniqueness guard on city coordinates. `_enforce_unique_city_coords`
  runs at the end of `place_cities` and deterministically perturbs any
  duplicate (row, col) via a fixed spiral walk to the first free
  walkable land cell. Rare in practice but the MST collapses to a
  zero-distance edge otherwise, producing an empty A* path and silently
  dropping the road.

- Grid header validation. `load_markers` now raises `AtlasGridMismatch`
  if the loaded `grid: {w, h}` header does not match `GRID_W`/`GRID_H`.
  Both the incremental-skip path and the regenerate path route through
  this loader, so a hand-authored template shipping a different grid
  size fails loud with a per-body error rather than silently producing
  half-scale coordinates.

- Unused `seed_rng` parameter removed from `_analyse_terrain`. The
  function is RNG-free (continent flood-fill, habitability scoring,
  river-mouth dedup, cost grid — all pure functions of terrain). The
  false API contract made it look like terrain analysis consumed RNG
  state and had to be sequenced with downstream RNG use.

- `_score_capital_sites` river-mouth bonus now builds one sparse
  accumulator with all mouth points set at once and runs a single
  `gaussian_filter` call, instead of O(n_mouths) filter calls over
  single-point images.

- `binary_dilation(analysis["land_mask"] == False)` replaced with the
  idiomatic `~analysis["land_mask"]`, matching the convention used
  elsewhere in the file.

- `atlas-generate` Makefile target now guards on
  `SELECT COUNT(*) FROM bodies WHERE terrain_reference IS NOT NULL`.
  On a fresh DB that count is 0 and the generator previously exited
  "success" after processing zero bodies. The target now fails loud
  with a pointer to `populate_terrain_reference.py`.

- `main.rs` SimRng defensive re-insertion gains a long comment
  explaining the exact plugin-ordering hazard it guards against, so
  future readers don't treat the line as dead code. Tied to #826.

Makefile

@@ -329,7 +329,18 @@ economy-db:  ## Import economics data (commodities, chains, gate links) into sys
 	@python3 tooling/economy-db/import_economics.py
 
 atlas-generate:  ## Generate atlas markers (cities, roads, rail) for all inhabited bodies (#832)
-	@cd tooling/planet-gen && python3 generate_atlas.py --seed 42
+	@# Loud guard: generate_atlas.py reads bodies with a non-NULL terrain_reference.
+	@# If populate_terrain_reference.py has not run on a fresh DB, the generator
+	@# silently processes zero bodies and exits 0 — fail fast instead.
+	@count=$$(python3 -c "import sqlite3; c = sqlite3.connect('server/data/systems.db'); print(c.execute('SELECT COUNT(*) FROM bodies WHERE terrain_reference IS NOT NULL').fetchone()[0])"); \
+	if [ "$$count" = "0" ]; then \
+	    echo "ERROR: no bodies have terrain_reference populated yet."; \
+	    echo "Run: python3 tooling/planet-gen/populate_terrain_reference.py"; \
+	    echo "(This is a prerequisite for atlas-generate — see D-191 §9 pipeline order.)"; \
+	    exit 1; \
+	fi; \
+	echo "  [guard] $$count bodies with terrain_reference — proceeding."
+	@python3 tooling/planet-gen/generate_atlas.py --seed 42
 
 econ-sim:  ## Build the economics simulation binary (Layer 1+2: Leontief + tâtonnement trade)
 	@cargo build --manifest-path tooling/econ-sim/Cargo.toml --release