feat: richer nebula sky and planet surface textures

Extend gen_nebula_sky.py with two more dust-lane layers at different
scales and subtle hue variation within the bright core. Add
gen_planet_surface.py (no prior generator existed) producing latitude
bands, storm vortices, and a lit/unlit terminator baked from the
planet mesh's actual UV convention against arena_02's directional
light, verified in-engine via screenshots.

TODO.md

@@ -41,7 +41,7 @@ A subagent ran the game and critiqued arena_02 head-on against Rocket League 2 i
 - [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.
+- [x] **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. Done: `tools/textures/gen_nebula_sky.py` gained two more dust-lane layers at different scales plus subtle patchy hue variation within the bright core; `tools/textures/gen_planet_surface.py` is a new generator (no prior script existed, despite the note below) producing latitude bands, 3 storm vortices, and a lit/unlit terminator baked from the sphere's exact UV convention (reverse-engineered from `nebula_planet.glb`'s vertex data) dotted against `arena_02.tscn`'s actual `DirectionalLight3D` direction — verified in-engine via godot-mcp screenshots showing a clean crescent terminator; both `Game/assets/textures/planet_surface.png` and the live-feeding sidecar `Game/assets/models/nebula_planet_planet_surface.png` were regenerated and reimported.
   Prompt: "Extend the nebula/planet texture generators (see `tools/textures/` once committed, per the star-sprite TODO item above) with more detail layers: for the sky, add a second/third dust-lane layer at a different scale plus subtler color variation within the bright core (real nebulae aren't one flat color); for the planet, add a proper lit/unlit terminator gradient (the side facing the arena's directional light should read brighter), more band variation at different latitudes, and a couple more storm-vortex features so it doesn't read as a single gradient with one twist. Regenerate both textures, reimport, and screenshot-verify."
 
 - [ ] **Particle lighting response for the nebula dust ("weather") effect** (no models — particle material/shader only). The `NebulaDust` `GPUParticles3D` motes are generic soft glow sprites with no lighting interaction, no depth-based fade, and no secondary motion (no sparkle/color shift).