[Purdze]

chunk meshes go in budgeted arenas now, far chunks get evicted when vram runs out instead of leaving holes

• arenas are budgeted from VK_EXT_memory_budget (a quarter of their heaps without it); an arena the driver can't allocate holds off adding more for a while, longer each time, instead of crashing
• eviction goes farthest first behind a frontier that resets once nothing waits; a column handed back to evict for holds the room until it uploads, so free space split into small gaps can't bounce two columns back and forth
• empty arenas compact and free once nothing waits for room; when a waiting column has room in total but no gap for it, the emptiest arena drains into the others' gaps for it
• pomme-gpu-allocator keeps one spare empty block per memory type instead of freeing it, for every allocation, not just arenas

windowed chunk bench, 2 runs each, 1 warmup + 3 measured per run:

• rd 32: all 3725 chunks in 1.62 s (11 arenas, 1936 MB); master gets ~2877 in 7.3-8.0 s and skips the rest (vertex pool full)
• rd 64: all 13881 chunks in 5.4 s (31 arenas, 5456 MB); master gets ~2900 in 13-14 s
• worst frame the same as master (17 vs 17-18 ms at rd 32, 25 vs 25-26 ms at rd 64)

tests: unit tests for the allocator, eviction, retry and frontier, plus a randomized streaming test against a model of the store; ingame scripts chunk_arenas, chunk_arenas_teleport, render_distance