Add TODO with roadmap for audio, large maps, spacecraft art, and asteroid polish

TODO.md

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+# TODO
+
+## Distance-based sound effects
+Sounds (explosions, impacts, etc.) should attenuate based on distance from the
+player. Offscreen events far away should be inaudible. Add a helper that takes
+a world position and scales volume by proximity to the camera/player.
+
+## Large map support (5000x5000)
+Audit the engine for anything that breaks or becomes slow at very large map
+sizes. Key areas to check:
+- Tile layer allocation (`uint16_t *` for 25M tiles)
+- Tilemap rendering culling (already viewport-clipped, verify correctness)
+- Physics / collision queries (should only check nearby tiles)
+- Entity updates (currently iterates full pool regardless of distance)
+- Camera bounds and coordinate overflow (float precision at large coords)
+- Level file parsing (row lines could exceed fgets buffer at 5000+ columns)
+
+## Spacecraft art and entity
+Create placeholder pixel art for the player's spacecraft (fits the 16x16 grid
+aesthetic but spans multiple cells, roughly 4-5 tiles wide by 2-3 tiles tall).
+Needs at minimum:
+- Landed/idle frame (visible near spawn on planet surface levels)
+- Landing animation (descending + touchdown, 3-4 frames)
+- Takeoff animation (liftoff + ascending, reuse landing in reverse)
+Used in two contexts:
+- **Level start**: ship flies in, lands at spawn, player exits, ship takes off
+  and leaves — serves as the level intro sequence
+- **Level exit**: ship flies in and lands when player nears the exit zone,
+  player enters, ship takes off — triggers level transition
+
+## Asteroid refinement
+- Reduce asteroid count significantly — occasional hazard, not a barrage
+- Add slight horizontal drift so they fall at an angle rather than straight down
+- Increase fall speed for more impact
+- Stagger spawns more widely across the level