[Engine] Overhaul simulation correctness, world generation, and chunk sleeping

[Kakapio]

Summary

A deep-dive review of the core engine found several correctness bugs (including one genuine data race), constant GPU re-upload churn, and a move pipeline that every future particle type would have to re-implement. This PR fixes all of it in three pieces, with a regression test suite to keep it fixed.

1. Move pipeline: one pure decision path, one application path

Simulator implementations are now pure: FluidSimulator::calculate_step reads a StepContext and returns a StepAction (Stay, MoveTo, ReplaceTarget, ConvertTarget) — the engine owns all writes via a single apply_local_step. In-chunk empty-cell moves apply immediately; cross-chunk moves and all interactions become DeferredSteps, applied serially in deterministic order with both ends re-validated. A step that lost its race simply doesn't happen and the particle stays put.

This one mechanism fixes four bugs:

• Cross-chunk Replace interactions placed the result at the source cell (water turning into obsidian in place, lava untouched)
• Cross-chunk Preserve interactions silently did nothing
• Inter-chunk move conflicts destroyed the losing particle (slow mass leak wherever streams converged near chunk borders)
• In-chunk interactions only fired when the target happened to be earlier in iteration order

Simulation is now fully deterministic: per-(tick, chunk) seeded SmallRng plus the ordered apply phase make thread scheduling irrelevant. The dashmap dependency is gone.

2. World generation: no more unsafe, seeded, layout-correct

The old generator shared a Vec<Chunk> across manually-spawned threads through UnsafeCell + unsafe impl Sync, partitioned by column ranges — but ore veins write at ±1 offsets across partition boundaries, so this was a real data race (UB). It's replaced by rayon per-chunk generation with vein spill-over collected and applied serially. Zero unsafe remains in the codebase.

• Map::generate_with_seed makes worlds reproducible (generate still picks a random seed)
• Chunks now live in a flat row-major Vec behind one shared coords::chunk_index convention, fixing a latent bug where the chunk layout was only correct for square maps
• roll_special_particle no longer allocates a Vec and sorts per cell
• Generation of the full 640×640 map runs in ~0.9 ms

3. Chunk sleeping and conditional versions

• Chunk::version only bumps when a pass actually changes something, so the renderer stops re-uploading 4 KB materials at 80 Hz for still water
• A chunk whose pass changes nothing goes to sleep; it wakes on any mutation (set_particle_at, including facing neighbors for border cells) or when a neighbor's border changes (BorderActivity). Simulation cost now scales with moving liquid, not total liquid
• The old dirty/trigger_refresh/update_dirty_chunks machinery became redundant and was deleted — painting now activates simulation without depending on Update-schedule ordering

Tests

Restructured as lib + bin (src/lib.rs) so integration tests use cavernborn:: directly instead of the previous #[path] include hack. 24 tests:

• Mass conservation: particle count asserted on every one of 300 ticks while a blob falls and spreads across chunk boundaries
• Cross-chunk and in-chunk interaction regressions (obsidian forms at the lava's position; acid conversion preserves the water)
• Sleep/wake: settled chunks stop simulating and their versions freeze; erasing a particle wakes them
• Determinism: identical worlds after 150 ticks of chaotic spreading
• Generator: seed determinism, no floating gaps, plus insta snapshots of generated terrain and a settled pool (64 particles → exactly two flat rows)

Deliberately preserved quirks (follow-up candidates)

• Liquids check only the target cell, not the path, so they can "tunnel" through thin floors at high viscosity (pre-existing)
• The .max(0) edge clamp lets lone droplets wander sideways along the floor row (pre-existing)
• Preserve interactions are directional despite commutative rule keys — part of the planned material/property-table redesign

🤖 Generated with Claude Code