feat(fileio): port async pools to Rust

Replace programs/fileio_asyncio.c with a declaration-only shim and register
Rust's async file-I/O implementation in the library archive. Preserve the C
ABI for pool and job contexts, including DEBUGLEVEL pointer-based pthread
wrappers, and select separate Rust build configurations for those layouts.
Keep ordered-read queue state locked while waiting, drain queued work before
shutdown frees job buffers, and use platform large-file seeks for sparse writes.

Test Plan:
- `cargo test --no-default-features --lib fileio_asyncio` (7 passed).
- `cargo test --no-default-features --features debug-pthread --lib fileio_asyncio` (7 passed).
- `cargo check --all-targets`, clippy library/benches/tests, and nightly fmt check (passed).
- Threaded program build and README round-trip (passed).
- `make -C tests poolTests` and `./tests/poolTests` (passed).
- Current C shim object defines no `AIO_*` symbols; the Rust archive exports all 20 declarations.
- Full standalone all-target Rust tests and the default CLI archive rebuild remain affected by unrelated C/Rust integration and another worker's in-progress benchmark changes.

rust/Cargo.toml

@@ -13,6 +13,8 @@ decompression = []
 dict-builder = []
 huf-force-decompress-x1 = []
 huf-force-decompress-x2 = []
+# POSIX DEBUGLEVEL >= 1 replaces pthread objects in the C ABI with pointers.
+debug-pthread = []
 # Legacy-format decoders (zstd v0.1 .. v0.7). Never default features: the
 # build systems map ZSTD_LEGACY_SUPPORT=N to the features for versions >= N,
 # exactly mirroring which lib/legacy/zstd_v0N.c files the C build compiles.