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.

server/src/main.rs

@@ -176,8 +176,18 @@ fn main() {
     app.insert_resource(BridgeResource::new(bridge));
     app.insert_resource(HandshakeState::Complete);
 
-    // SimulationPlugin { seed } already inserts SimRng with the correct seed.
-    // Re-insert here as a defensive override in case plugin ordering shifts.
+    // SimulationPlugin { seed } already inserts SimRng with the correct seed
+    // during plugin build. We re-insert here as a defensive override for one
+    // specific ordering risk: any future plugin that registers *before*
+    // SimulationPlugin in `App::add_plugins` order (e.g. a pre-simulation
+    // observability plugin) and consumes SimRng at plugin build time would
+    // see a stale resource that was never seeded from StartupMessage. This
+    // `insert_resource` call happens AFTER all plugins have built, so it
+    // always overwrites whatever SimRng is currently in the world with the
+    // authoritative value from the StartupMessage. If you remove this line,
+    // also audit every `app.add_plugins(...)` call in this file and in
+    // `SimulationPlugin::build` for plugins that touch SimRng, and verify
+    // none of them run before SimulationPlugin's seeding logic. #826 thread.
     app.insert_resource(settled_reach_server::simulation::rng::SimRng::new(seed));
 
     // Initialize empty line pool index (populated by generator pipeline in v0.2).