docs(fast_io): map io_uring opcodes to IOCP equivalents (WPG-7.b)

[oferchen]

Summary

• Maps every io_uring opcode catalogued in WPG-7.a (docs/design/wpg-7-iouring-opcode-inventory.md, merged as docs(fast_io): inventory io_uring opcodes (WPG-7.a) #4755) to its Windows IOCP / Winsock / RIO equivalent or an explicit gap.
• Covers all 23 opcodes in use: 15 SQE-side rows (READ/WRITE, READ_FIXED/WRITE_FIXED, FSYNC, SEND/SEND_ZC/RECV, POLL_ADD, STATX, RENAMEAT, LINKAT, ASYNC_CANCEL classic + extended, LINK_TIMEOUT, NOP) and 8 register/setup rows (REGISTER_FILES, UNREGISTER_FILES, REGISTER_BUFFERS, UNREGISTER_BUFFERS, REGISTER_PROBE, REGISTER_PBUF_RING, UNREGISTER_PBUF_RING, SETUP_SQPOLL).
• Classifies each row by severity (P0 data-path, P1 metadata, P2 utility, N/A) and identifies 4 concrete gaps: NOP, READ_FIXED, WRITE_FIXED, LINK_TIMEOUT.
• Cross-references feed WPG-8 (zero-copy send: SEND_ZC + SEND rows) and WPG-9 (registered buffers: READ_FIXED, WRITE_FIXED, REGISTER_BUFFERS, UNREGISTER_BUFFERS, REGISTER_PBUF_RING, UNREGISTER_PBUF_RING rows).
• Audit-only - no source changes.

Test plan

• Doc-only change; CI fmt+clippy/nextest/Windows/macOS/Linux musl checks should pass unchanged.