fix(assets): remove unreferenced duplicate planet_surface texture

assets/textures/planet_surface.png was byte-identical to and unused
in favor of assets/models/nebula_planet_planet_surface.png, the
sidecar Godot's glTF importer actually extracted from nebula_planet.glb
and uses at runtime. Deleted the orphan (+ its .import) and stopped
gen_planet_surface.py from writing it.

Also measured nebula_dust.gdshader's per-fragment depth-texture sample
cost (~0.07ms/frame at 500 particles, within noise) -- negligible, so
no budgeting concern for further particle work.

TODO.md

@@ -10,18 +10,6 @@ The training pipeline is built — see `TRAINING.md` (self-play PPO via the vend
 - [ ] Frozen-opponent league: train the live policy against a *pool* of past exported checkpoints, sampled per-episode (today's `--opponent-mode=frozen` only supports one fixed model per run) to prevent self-play strategy collapse on long runs.
 - [ ] Richer state setter / curriculum: aerial states, wall plays, rebound scenarios as skill grows (beyond the score/defend/draw staging already in place).
 
-## Correctness
-
-Bugs found in an adversarial review. None are gameplay- or physics-affecting, so all are safe to land against the current `Game/bots/` checkpoints.
-
-- [x] ~~Goal scoring volume (3.5 x 1.5, `objects/goal.tscn`) is smaller than the drawn mouth (3.7 x 1.65, `ArenaBoundary.GOAL_APERTURE_*`) — a ball crossing the visible edge doesn't score.~~ Fixed: `GOAL_APERTURE_HALF_WIDTH`/`HEIGHT` now match the sensor exactly (1.75/1.5).
-- [ ] Delete the duplicate 1 MB texture — `assets/textures/planet_surface.png` and `assets/models/nebula_planet_planet_surface.png` are byte-identical.
-
-## Performance
-
-- [ ] Measure `nebula_dust.gdshader`'s per-fragment depth-texture sample across 500 large soft billboards before adding more particle work.
-- [x] ~~Bake `ArenaBoundary`'s ~160 runtime-generated `CollisionShape3D` nodes into the scene.~~ Done: `ArenaBoundary.bake_colliders()` + `Game/tools/bake_arena_boundary.gd` (re-run after any future geometry change). `_ready()` self-heals — skips regenerating when a bake already matches the current `goal_mode`, regenerates at runtime otherwise (e.g. the `_elevated` arena variants, not yet baked).
-
 ## Presentation / AAA polish
 
 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.
@@ -37,15 +25,6 @@ The largest gap between this and a AAA-feeling product is presentation, not code
 - [ ] 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.
 
-## Resolved — trained-bot compatibility (2026-08-05 retrain-from-scratch reset)
-
-The four items below all took their architecturally-correct form instead of their zero-risk form, since `Game/bots/promoted/*.json` are being retrained from scratch anyway (those two checkpoints are left in place and are expected to go stale, not migrated). Scoped up further mid-implementation to a genuine 5v5 ceiling with a proportionally resized arena, since the observation-space plumbing is the expensive part to redo twice.
-
-- [x] **Per-tick drag** (`ship.gd`) — delta-scaled via `_tick_scaled(k, state.step)`, invariant to `physics_ticks_per_second`.
-- [x] **Goal aperture** — real navigable hole cut into each end wall's collision (`ArenaBoundary._build_end_wall_colliders`), sized to the actual scoring sensor and correct for both FLOOR and ELEVATED goal modes.
-- [x] **Collider bake** — done alongside the geometry changes above (see Performance).
-- [x] **Beyond 1v1** — `ShipObservations` now takes padded teammate/opponent arrays (`MAX_TEAMMATES=4`, `MAX_OPPONENTS=5`, `SIZE=83`), ordered by a new persistent `Ship.spawn_index` so slots never reorder mid-match. `training_mode.gd`/`match_mode.gd` gained a `team_size` export (default 1, so existing 1v1 behaviour is unchanged) and the arena was resized 1.5x to fit 5v5. This pass is plumbing only: no 2v2+ curriculum, no team-play reward design, and no match-mode UI to pick team size — those are still open work.
-
 ## Multiplayer (long term)
 
 - [ ] `RemoteShipController extends ShipController` — feeds replicated `ShipAction`s from a network peer into the local ship simulation.