Fix GWP-ASan usable-size expectation

[Matthew Parkinson (mjp41)]

Summary

• distinguish primary snmalloc allocations from secondary allocations in the malloc usable-size test
• retain exact rounded malloc_good_size expectations for unaligned primary allocations
• require secondary allocations to provide at least the requested usable size without assuming exact equality

Validation

• formatted src/test/func/malloc/malloc.cc
• git diff --check
• fresh-context implementation review approved
• local GWP-ASan testing skipped because this checkout has no local GWP-ASan build; CI is the runtime validation source

This test-only fix is separate from #881.