chore(deps): switch mimalloc3 to upstream microsoft/mimalloc

[shulaoda]

Background

On macOS, mimalloc v3 stores the per-thread heap pointer in a fixed TLS slot (TCB [108]/[109]) shared by every image, so a second statically-linked copy (e.g. another napi addon) adopts the first's heap and crashes. We worked around this with a vendored prim.h patch in the napi-rs/mimalloc fork. Upstream now fixes it in microsoft/mimalloc#1301 (b83dee64) by exposing cmake options, so we can drop the fork.

Changes

• .gitmodules — mimalloc3 url napi-rs/mimalloc → microsoft/mimalloc.
• mimalloc3 — 5853e09 (fork) → b83dee64 (official dev3); drops the vendored prim.h patch.
• build.rs — on v3 + macOS, enable the official options:

  if env::var_os("CARGO_FEATURE_V3").is_some() && target_os == "macos" {
      cmake_config
          .define("MI_TLS_MODEL_LOCAL", "ON")     // -> MI_TLS_MODEL_THREAD_LOCAL=1
          .define("MI_TLS_RECURSE_GUARD", "ON");  // -> MI_TLS_RECURSE_GUARD=1
  }

• mimalloc (v2) — routine bump 02a2f5d → fef6b0d.

Notes

• Scope: v3 + macOS only. Options exist only in v3; Linux already defaults to thread-local (no-op), Android/OpenBSD use pthreads, Windows uses its own TlsAlloc path. Add target_os == "ios" to cover iOS/tvOS.
• MI_TLS_RECURSE_GUARD is belt-and-suspenders — upstream prim.h already auto-enables it on Apple; we set it explicitly for clarity.
• No regression of the dead-main-thread fix (issue #1287 / rolldown#9722): the MI_THREADID_INVALID poison in mi_tld_free is present in b83dee64.

Verification (macOS)

• v3 build: CMakeCache MI_TLS_MODEL_LOCAL:BOOL=ON / MI_TLS_RECURSE_GUARD:BOOL=ON; compiled with MI_TLS_MODEL_THREAD_LOCAL=1 + MI_TLS_RECURSE_GUARD=1.
• Default (v2) build: options absent (gating works), builds clean.
• cargo test --features v3: mimalloc-safe 6/6 pass; libmimalloc-sys-test runs.