feat(arena): add grounded lighting and asteroid dressing

TODO.md

@@ -15,13 +15,13 @@ The training pipeline is built — see `TRAINING.md` (self-play PPO via the vend
 The largest gap between this and a AAA-feeling product is presentation, not code. Sequenced after the above for pragmatic reasons, but this is the highest impact per hour.
 
 - [ ] **Audio — there is none.** Zero sound files, zero `AudioStreamPlayer` nodes, no bus layout. Needs: engine hum pitched to throttle, turbo whoosh, ball impacts scaled by collision impulse, wall scrapes, goal explosion, crowd bed, UI clicks, countdown beeps, music. Can be driven off `Ship`'s existing telemetry signals.
-- [ ] SSAO/SSIL in the arena Environments — cheapest single perceived-quality win available; grounds the ships against the deck and gives the fillets and goal recesses real depth.
+- [x] SSAO/SSIL in the arena Environments — cheapest single perceived-quality win available; grounds the ships against the deck and gives the fillets and goal recesses real depth.
 - [ ] VFX on anything that moves: `EngineGlowL/R` are static meshes that don't respond to throttle. No thruster plume, turbo flame, ball trail, impact sparks or goal burst. `arena_02` is the only scene with any particles at all.
 - [ ] Impact feedback — screen shake, hit-stop, flash, controller rumble on ball contact.
 - [ ] Camera feel in `ship_camera.gd` — fixed distance/height/FOV today. Speed-based FOV widening, turbo kick, impact shake.
 - [ ] Goal celebration sequence: today it's a `print()` and a label scale-pop. Wants an explosion, team-tinted screen flash, camera cut, slow-mo, title card. `HUDController`'s `ResultOverlay` animation is a reasonable template.
-- [ ] Local lighting / LightmapGI bake — two directional lights and low ambient, no local lights anywhere. `arena_deck.gdshader`'s `hull_fill` uniform is explicitly a "stand-in for bounce light", i.e. the shader is compensating for lighting that isn't there. The arena is fully static, so a bake is viable; retire `hull_fill` afterwards.
-- [ ] Dress `arena_03` — its `Decoration` node is empty, while `arena_02` has stations, debris, a planet and volumetric dust.
+- [x] Local lighting / dynamic GI — four local pitch lights plus SDFGI provide bounce and contact lighting for the runtime-generated arena shell; the shader's `hull_fill` stand-in is retired. (The shell is generated in `_ready`, so it cannot participate in an editor LightmapGI bake.)
+- [x] Dress `arena_03` — its asteroid field now has mining stations, debris clusters and a distant planet.
 - [ ] Custom font + a real `Theme` resource for the HUD. The procedural instruments are well-engineered, but `ThemeDB.fallback_font` at 10-13 px reads as a debug overlay.
 - [ ] Post-processing beyond glow: DoF, motion blur, vignette, chromatic aberration on turbo.