phase(N.5b) Task 2: TerrainSlotAllocator + tests
Pure-CPU slot allocator for the terrain modern dispatcher's global
VBO/EBO. FIFO free-list + monotonic counter, mirroring WB's
TerrainRenderManager pattern. Caller (TerrainModernRenderer) handles
GPU buffer growth when Allocate sets needsGrow=true.
8 unit tests cover: fresh-allocator returns slot 0, sequential
allocs, free+alloc reuse, FIFO ordering, needsGrow signaling on
capacity overflow, GrowTo, LoadedCount tracking, and double-free
detection.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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