Chunk cell storage won't scale to open-world memory

[starzonmyarmz]

Problem

Chunk cells use Box<[Option<Color8>]> (src/grid.rs:29, Color8 = [u8; 4]).

[u8; 4] uses all 256 bit patterns per byte, so Option<[u8; 4]> has no niche — it needs a separate discriminant byte:

size_of::<Option<[u8; 4]>>() == 5

Storage is dense, not sparse: a chunk allocates the full CHUNK_VOL array on first write regardless of fill.

• 5 B × 32,768 (CHUNK_VOL) = 160 KB / allocated chunk
• × 4,000 (LARGE_SCENE_CHUNKS, src/grid.rs:15) = ~640 MB for cell arrays alone

This caps practical scene size well below an "open world" target.

Proposed fix (~4-5x cut)

Palette-indexed cells + separate occupancy bitset:

• palette: Vec<Color8> (per-grid or global) + chunk stores Box<[u8; CHUNK_VOL]> palette indices
• occupancy Box<[u64; CHUNK_VOL / 64]> bitset (512 B/chunk) replaces the Option tag
• get: bitset bit -> palette index -> color
• set: intern color, set bit + write index

Per chunk: 32,768 (u8 index) + 512 (bitset) = ~32.5 KB -> ~4.9x smaller. 640 MB -> ~130 MB.

Caveats / scope

• 256-color cap per palette with u8 index. If scenes exceed 256 distinct colors, need u16 index (2x) or per-chunk palettes. Roxel is already palette-oriented (palettes.ron).
• set's drop-on-empty, occupancy count, and seam-dirty logic all need rework (src/grid.rs:111-176).
• Touches mesher + picking read paths (they call get).
• Prototype behind the existing get/set API so callers don't change.

Priority

Flag, not emergency. No correctness bug today — purely a scaling ceiling.