Add opt-in MI_SINGLE_THREADED specialization for the free fast path

[Alan-S-Andrade]

Summary

In a program that only ever uses mimalloc from a single thread, every freed block is guaranteed to be thread-local, so the per-free thread-ownership check in mi_free_ex — a thread-id TLS read, a relaxed atomic load of segment->thread_id, and a compare — is redundant.

This PR adds an opt-in, off-by-default compile-time switch -DMI_SINGLE_THREADED=1 that forces is_local = true and skips that check on the free fast path. It is motivated by recent literature on specializing general-purpose allocators for single-threaded use (e.g. ExGen-Malloc, "Old is Gold", IEEE CAL'25).

#if defined(MI_SINGLE_THREADED) && (MI_SINGLE_THREADED)
  const bool is_local = true;
#else
  const bool is_local = (_mi_prim_thread_id() == mi_atomic_load_relaxed(&segment->thread_id));
#endif

Safety / default build

The default build is unchanged — the #else path is identical to the previous code. The flag is a single-thread-only promise by the embedder (multi-threaded use is out of contract, exactly like other single-thread allocator builds). With the flag off, the full test suite (including the multi-threaded test-stress and test-stress-dynamic) passes.

Measurements

mimalloc-bench, single-threaded, pinned to one core, interleaved median-of-11 (baseline = current dev):

benchmark	baseline	MI_SINGLE_THREADED	delta
alloc-test (1 thread)	4.62 s	4.60 s	-0.43%
sh6bench	1.67 s	1.66 s	-0.3%
cfrac	7.59 s	7.58 s	~0% (compute-bound)
espresso	6.24 s	6.23 s	~0% (compute-bound)

perf stat on alloc-test confirms the mechanism: ~2.5% fewer branches (~50M, the eliminated per-free ownership branch) and ~1.1% fewer instructions. cfrac/espresso stdout is byte-identical to baseline.

The gain is modest but consistent on allocation-heavy single-threaded workloads, with zero cost or risk to the default multi-threaded build.

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