Add opt-in MI_SINGLE_THREADED specialization for the free fast path

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 adds an opt-in, off-by-default compile-time switch
(-DMI_SINGLE_THREADED=1) that forces is_local=true and skips that check.
The default build is unchanged (the #else path is identical to before
and the full multi-threaded test suite still passes).

Measured on a single-threaded, allocation-heavy workload (mimalloc-bench
alloc-test, 1 thread, pinned, interleaved median-of-11): ~0.4% faster
wall time, with perf showing ~2.5% fewer branches and ~1.1% fewer
instructions (the eliminated per-free ownership branch). cfrac/espresso
output is byte-identical to baseline.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Alan-S-Andrade

src/free.c

@@ -153,7 +153,14 @@ static inline void mi_free_ex(void* p, size_t* usable) mi_attr_noexcept
   mi_segment_t* const segment = mi_checked_ptr_segment(p,"mi_free");
   if mi_unlikely(segment==NULL) return;
 
+  #if defined(MI_SINGLE_THREADED) && (MI_SINGLE_THREADED)
+  // Single-threaded specialization (ExGen-Malloc, CAL'25): in a program that only ever
+  // uses mimalloc from one thread, every block is thread-local, so we can skip the
+  // thread-id TLS read + atomic load + compare on every free.
+  const bool is_local = true;
+  #else
   const bool is_local = (_mi_prim_thread_id() == mi_atomic_load_relaxed(&segment->thread_id));
+  #endif
   mi_page_t* const page = _mi_segment_page_of(segment, p);
   if (usable!=NULL) { *usable = mi_page_usable_block_size(page); }