A racing collection can wrap a block's fragmented row count #92

When a collection starts, the collecting thread zeroes every other allocator's spaceEnd without a lock (WaitForSafe), so the allocation code must treat it as volatile. CallAlloc reads it twice when it adds an abandoned hole to the block's mFraggedRows: once for spaceEnd > spaceStart and again for spaceEnd - spaceStart. If the collector zeroes it in between, the difference is negative and the uint16 count wraps (seen as 65318 from a delta of -218). The count feeds the heap statistics that size working memory. test/gc_stress hits it on Linux in debug builds, where the row-count check traps.

Related: HaxeFoundation/hxcpp#1392 (same symptom; its guard does not cover the race), HaxeFoundation/hxcpp#1409.