chore(deps): update mimalloc3 submodule to latest dev3 (#78)

## What

Updates the `mimalloc3` submodule (napi-rs/mimalloc `dev3` fork) from
`9a15529` to `5853e09`.

The `dev3` branch was rebased, so the old Apple-patch commit was
replaced and several upstream commits are now pulled in:

- `fix(apple): use THREAD_LOCAL + RECURSE_GUARD instead of FIXED_SLOT`
- `allow initial main thread to terminate before the process terminates`
(upstream issue #1287)
- `remove wchar.h include from mimalloc.h`
- additional assertions / an assertion-failure fix

## Notes

- The `mimalloc` (v2.x) submodule stays at **v2.3.2**, which is still
the latest v2 release tag — no v2 change in this PR.

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: shulaoda