refactor: implement a faster file digest

[rgrinberg]

Do open/digest/close and return the file stat all in a single thread

[Alizter]

Does this mean we don't need the throttle for the digests and can instead move that to the thread pool?

[rgrinberg]

No, it should still stick around. Digesting is mostly CPU bound. We do need to lower it significantly though.

[rgrinberg]

I adjusted the throttle here as well.

[Copilot]

Pull request overview

Replaces the fiber-orchestrated file_async (which opened the fd in the main thread, then streamed via the async pool with throttling/empty-file/short-circuit logic) with a single C stub that does open/fstat/read/close entirely off the OCaml runtime, returning both the digest and size in one shot. The fiber wrapper is reduced to a throttled async_exn call. A Windows fallback in OCaml is provided.

Changes:

• New Blake3_mini.file_with_size C stub (Unix) + OCaml fallback (Windows) returning (Digest.t * int).
• Simplified Digest.file_async: throttle lowered from 100 to 32, no zero short-circuit, no async_digest_minimum, no main-thread open/fstat.
• Cram test helper dune_build_sorted_actions to sort build lines in repro-check.t, accommodating non-deterministic action ordering induced by the new digest path.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/ocaml-blake3-mini/blake3_stubs.c	Adds blake3_mini_file_with_size stub doing open/fstat/read/close with runtime released.
src/ocaml-blake3-mini/blake3_mini.ml	Binds new stub; provides Windows OCaml fallback selecting via Sys.win32.
src/ocaml-blake3-mini/blake3_mini.mli	Exposes file_with_size : string -> Digest.t * int.
src/dune_digest/digest.ml	Rewrites file_async to use the new combined stub; removes empty/small-file fast paths and stat-on-main-thread.
test/expect-tests/blake3/blake3_mini_tests.ml	Adds expect test for file_with_size.
test/blackbox-tests/test-cases/dune-cache/repro-check.t	Adds sorting helper to stabilize action order in expected output.