docs(fast_io): prioritised IOCP gap list (WPG-7.c)

[oferchen]

Summary

• Synthesise WPG-7.a (opcode inventory, docs(fast_io): inventory io_uring opcodes (WPG-7.a) #4755) and WPG-7.b (IOCP mapping, docs(fast_io): map io_uring opcodes to IOCP equivalents (WPG-7.b) #4760) into a prioritised gap list.
• Rank the four confirmed gaps (NOP, READ_FIXED, WRITE_FIXED, LINK_TIMEOUT) plus the WPG-8 hand-off (SEND_ZC) from P0 to P3, with severity rationale, user-visible impact, Win32 workaround sketch, effort estimate, and follow-up owner.
• File new follow-ups: WPG-10 (Windows linked-timeout shim) and optional WPG-11 (IOCP test harness parity). WPG-9 owns the P0 file-side registered-buffer gap; WPG-8 owns the SEND_ZC delta.
• Recommend sprint sequencing: WPG-9 first (the only P0), then WPG-8, then WPG-10, with WPG-11 contingent on WPG-9's test plan.

Audit-only doc, no source changes.

Test plan

• CI fmt+clippy (no source changes, no-op verification).
• Markdown renders cleanly in the GitHub diff view.
• Cross-references resolve to existing files (docs/design/wpg-7-iouring-opcode-inventory.md, docs/design/wpg-7b-iouring-iocp-mapping.md).