feat: greeble/detail pass on nebula station and debris models

Add tools/blender/gen_nebula.py (previously nebula_decoration.blend had
no generator script, unlike ship/ball) to rebuild the station with
inset/extrude panel-line greeble and three separate emissive window
strips, and give debris its own rockier, damage-scarred materials
instead of cloning the station's hull material.

TODO.md

@@ -38,7 +38,7 @@ A subagent ran the game and critiqued arena_02 head-on against Rocket League 2 i
 - [x] **Real PBR lighting + materials, fresnel glass boundary**: SDFGI enabled and a `ReflectionProbe` added to all three arenas (`arena_01`/`02`/`03.tscn`); `arena_boundary.tscn`'s shared `StandardMaterial3D_field` converted from unshaded to shaded (roughness 0.05, rim-enabled fresnel highlight, alpha still near-invisible face-on). Verified via godot-mcp screenshots (60 FPS / 17ms frame time, no measurable cost) and zero-error headless runs of `free_play`/`match`/`spectate`/`training`.
   Prompt: "In Cosmic Clash, enable SDFGI and add reflection probes to the arena scenes (start with `arena_02.tscn`) so surfaces get real bounce lighting/reflections instead of flat ambient. Convert `arena_boundary.tscn`'s `StandardMaterial3D_field` from unshaded to a proper shaded material with a fresnel-based rim highlight (bright at grazing angles, near-invisible face-on) so it reads as glass rather than a tinted overlay — check the performance impact of moving it off unshaded, given it's a large always-visible surface. Verify visually via godot-mcp screenshots, and confirm headless runs (`free_play`/`match`/`spectate`/`training`) still show zero errors."
 
-- [ ] **Greeble/detail pass on the station + debris models** (Blender remodel — explicit models to redo). `nebula_station.glb`/`nebula_debris.glb` are unmistakably Blender primitive kitbashes up close: plain boxes/cylinders with bevel modifiers, no panel lines, damage decals, or surface detail, and debris chunks look undifferentiated from the station.
+- [x] **Greeble/detail pass on the station + debris models** (Blender remodel — explicit models to redo). Rebuilt via a new committed generator, `tools/blender/gen_nebula.py` (`nebula_decoration.blend` had no prior script, unlike ship/ball — this brings it in line): the station gets panel-line inset/extrude greeble across three passes and 3 separate emissive window strips (was 1), and debris gets its own `Mat_DebrisRock`/`Mat_DebrisScorch` materials plus per-vertex jitter and impact-crater gouges instead of cloning the station's `Hull_Metal_Dark`. The full normal-map/AO bake was skipped as too fragile to script reliably (per this item's own fallback allowance) — detail is geometry + material-only, no trim texture needed on top.
   Prompt: "Using the blender MCP, rebuild `nebula_station` (source in `Game/assets/blender_models/nebula_decoration.blend`) with actual surface detail: greebled panel-line insets via bmesh inset-and-extrude on selected faces (more scriptable and robust than chained boolean cuts — prefer this over booleans for the main detailing pass), and 2-3 emissive window/light strips distinct from the existing single accent strip. A full normal-map/AO bake pipeline (low-poly + high-poly pair, UV unwrap, bake settings) is the ideal AAA-style finish but is fragile to script end-to-end in one pass — attempt it, but if it proves too unreliable, fall back to the inset/extrude geometric detail alone plus a simple tiled trim-sheet-style texture rather than forcing a bake that doesn't work. Give `nebula_debris` chunks a rockier/damaged material distinct from the station's clean hull (darker, rougher, maybe scorch-mark variation) so they read as separate debris rather than clones of the station's material. Re-export both to `Game/assets/models/nebula_station.glb`/`nebula_debris.glb`, re-save the shared `.blend`, and re-verify placement/transforms in `arena_02.tscn` still look right (screenshot check)."
 
 - [ ] **Richer nebula sky + planet surface textures** (extends the existing procedural generation — no new asset types). `sky_nebula.png` is essentially one noise-filter pass over a bright core; `planet_surface.png` is a flat gradient with a single vortex swirl and no bands, craters, or day/night terminator.