feat(fast_io): cache PBUF_RING probe and surface in --version

[oferchen]

Summary

Implements task #2043: probe IORING_REGISTER_PBUF_RING (Linux 5.19+) at runtime, cache the result behind OnceLock, surface it on IoUringKernelInfo, and document the layered fallback chain in code and audit docs.

• Adds a process-wide OnceLock-cached pbuf_ring_supported() -> bool probe in crates/fast_io/src/io_uring/buffer_ring.rs, re-exported at the crate root as fast_io::pbuf_ring_supported. The first call performs a uname(2) parse via config_detail::get_kernel_release_string; subsequent calls are a single relaxed atomic load.
• Surfaces the probe on IoUringKernelInfo::pbuf_ring_supported (both Linux and the cross-platform stub) and appends , pbuf_ring=yes/no to the IoUringProbeResult::Available reason string so --version output communicates which fallback tier is active.
• Cross-platform stub (io_uring_stub.rs) mirrors the same surface and always returns false; matching re-export added.
• Module-level rustdoc updated on io_uring/buffer_ring.rs and io_uring/mod.rs, and docs/audits/iouring-pbuf-ring.md updated with the Phase 2 status note.

Fallback chain

1. PBUF_RING (Linux 5.19+, IORING_REGISTER_PBUF_RING, opcode 22) - completion-time buffer selection via ring-mapped supplied buffers.
2. Classic provide-buffers (Linux 5.6+, IORING_OP_PROVIDE_BUFFERS opcode 31, or IORING_REGISTER_BUFFERS opcode 0) - pre-registered pool with per-SQE selection.
3. Standard read(2) / write(2) - traits::StdFileReader / traits::StdFileWriter fallback used when io_uring is unavailable entirely.
4. Non-Linux io_uring stub (io_uring_stub.rs) - pbuf_ring_supported() returns false; BufferRing::new returns Err(Unsupported); BufferRing::try_new returns None.

Test plan

New crates/fast_io/tests/pbuf_ring_probe.rs adds five integration tests covering:

• Idempotency across repeated calls (OnceLock caching contract).
• Stub returns false on non-Linux / feature-disabled builds; type-correct on Linux.
• pbuf_ring_supported() => is_io_uring_available() invariant (5.19+ implies the 5.6+ floor).
• IoUringKernelInfo::pbuf_ring_supported mirrors the crate-root probe.
• Cached probe returns identical values from 16 threads x 64 calls each (no torn reads, no re-entry into the kernel-version parser).

CI: fmt+clippy, nextest (Linux probe path; non-Linux stub path); audit doc renders.

Refs task #2043.