docs(audits): io_uring fixed-buffer registration audit

[oferchen]

Summary

• Adds docs/audits/io-uring-fixed-buffer-audit.md covering IORING_REGISTER_BUFFERS registration sites, READ_FIXED / WRITE_FIXED vs unfixed opcode usage, the RegisteredBufferGroup lifecycle (allocate, register, checkout, return, deregister), pool sizing decisions and presets, three deliberate gaps where fixed buffers are not used today (SharedRing registers but does not consume; IoUringDiskBatch has no group; socket I/O is unfixed and SEND_ZC is gated), the relationship to IORING_REGISTER_PBUF_RING, and bgid namespace usage.
• Cross-references the existing io-uring-adaptive-buffer-sizing, io-uring-bgid-namespace, iouring-pbuf-ring, and disk-commit-iouring-batching audits so the fixed-buffer story has a single authoritative entry point.
• Closes the io_uring fixed-buffer review item (Use release tarball for interop harness upstream builds #2118).

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --no-deps -- -D warnings
• CI required checks: fmt+clippy, nextest (stable), Windows, macOS, Linux musl