feat: lit particle shader for nebula dust weather effect

Replace NebulaDust's flat unshaded glow material with a custom
ShaderMaterial: a fake per-pixel puff normal on the billboarded quad
feeds a light() override so motes catch the directional light and a
nebula-core color bias, alpha gets a depth-texture soft-particle fade
so motes no longer hard-clip through boundary/decoration geometry,
and a per-particle hash adds subtle sparkle.

TODO.md

@@ -44,7 +44,7 @@ A subagent ran the game and critiqued arena_02 head-on against Rocket League 2 i
 - [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).
+- [x] **Particle lighting response for the nebula dust ("weather") effect** (no models — particle material/shader only). `NebulaDust`'s `StandardMaterial3D` was replaced with a new `ShaderMaterial` (`Game/shaders/nebula_dust.gdshader`, the project's first particle/spatial shader): a fake per-pixel puff normal is derived from local UV on the true-billboarded quad and fed into a `light()` override (`LIGHT`/`LIGHT_COLOR`/`ATTENUATION`) so motes visibly catch the arena's directional light and pick up a fixed nebula-core color bias; alpha is multiplied by a depth-texture-based soft-particle fade so motes no longer hard-clip through boundary/decoration geometry; a per-particle flicker (hashed from `INSTANCE_CUSTOM.x` + `TIME`) adds subtle sparkle. Verified via godot-mcp screenshots from multiple angles (visible lit/unlit shading variation, smooth fade near geometry, per-mote sparkle) and zero-error headless runs of `free_play`/`match`/`spectate`/`training`.
   Prompt: "In `arena_02.tscn`'s `NebulaDust` particle system, replace the current unshaded glow-sprite material with a custom shader that fakes lighting response — note the sprites are camera-facing billboards, so they have no fixed world-space normal and true per-pixel PBR lighting won't behave like it would on a solid mesh. A practical approach: derive a fake per-pixel normal from the quad's local UV (as if each sprite were a small sphere/puff, same trick used for 2D lit particle effects) and light that against the directional light + a fixed 'glow color' bias toward the nebula core direction, rather than relying on the mesh's real (camera-facing) normal. Add soft-particle depth fade (compare particle depth to the depth buffer) so motes don't hard-clip through the boundary/decoration geometry, and add subtle per-particle brightness flicker via the color ramp or shader for sparkle. Verify visually via godot-mcp screenshots that the dust reads as lit rather than flat-glowing, from a couple of different camera angles (billboard lighting tricks can look wrong from some angles even when right from others)."
 
 - [x] **Ship + ball visual pass**: `Game/objects/ship.tscn`'s 5 primitives replaced with a Blender-greebled hull/nose/canopy/tailfin/twin nacelles (source `Game/assets/blender_models/ship.blend`, generator `tools/blender/gen_ship.py`); ball fully remodeled as a smooth round icosphere with a crossed emissive accent pattern (`ball.blend`, `gen_ball.py`) rather than just a material tweak, replacing the old flat-shaded `gold_ball`. `Nose`/`TailFin` node names preserved for `_apply_team_color()`; `CollisionShape3D`/`RigidBody3D` physics on both ship and ball untouched (verified). Each part's mesh is extracted to a standalone `.res` (`tools/blender/extract_meshes.gd`) rather than referenced via `glb::ArrayMesh_xxx`, which doesn't reliably resolve across scene files. Verified via in-game screenshots (both team colors, orientation) and headless runs of `free_play`/`training`/`match`.