Bitmap allocations of at most 64 chunks only searched within a single
bitmap word. Contiguous free space crossing a word boundary was therefore
invisible and could force an unnecessary page growth. Larger requests
could also advance past the final bitmap before returning out of memory.
Search adjacent words for small cross-boundary runs and guard the final
word in the large-allocation path. Add boundary regressions and a
randomized oracle that verifies both placement and bitmap mutation.
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