chore: update mimalloc to latest dev3 (#26519)

## Summary
- Updates oven-sh/mimalloc bun-dev3 branch to latest upstream
microsoft/mimalloc dev3 (ffa38ab8)
- Merged 12 new commits from upstream

### Key upstream changes included:
- fix re-initialization of threads on macOS
- add lock for sub-pagemap allocations
- fix peak commit stat
- fix use of continue in bitmap find_and_clear (fixes rare case of not
finding space while it exists)

## Test plan
- [ ] CI passes
- [ ] Memory allocation tests pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

cmake/targets/BuildMimalloc.cmake

@@ -4,7 +4,7 @@ register_repository(
   REPOSITORY
     oven-sh/mimalloc
   COMMIT
-    989115cefb6915baa13788cb8252d83aac5330ad
+    ffa38ab8ac914f9eb7af75c1f8ad457643dc14f2
 )
 
 set(MIMALLOC_CMAKE_ARGS