perf(shirabe): use mimalloc as the global allocator

Profiling `create-project` showed glibc malloc/free accounting for about
41% of samples: the work is dominated by a very large number of small,
short-lived allocations, mostly from decoding repository JSON into
PhpMixed.

`create-project laravel/laravel`, offline against a warmed cache, all
three variants measured in one hyperfine run (8 runs each, identical
composer.lock):

  glibc      3.633 s +- 0.023   (user 3.580 s, sys 2.255 s)
  mimalloc   3.096 s +- 0.054   (user 3.004 s, sys 2.315 s)   -14.8%
  jemalloc   3.177 s +- 0.028   (user 3.136 s, sys 2.285 s)   -12.5%

The gain is entirely in user time; system time is unchanged, so the
extraction and file-writing half of the run is unaffected. Peak RSS drops
slightly, from 267 MB to 257 MB. mimalloc wins over jemalloc and, unlike
jemalloc, needs no C build of its own at every profile.

The allocator is declared in the library rather than in `main.rs` so that
the test and benchmark binaries, which link this crate but not `main.rs`,
run against the allocator the binary ships with.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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