docs(fast_io): audit SEND_ZC equivalent on Windows (WPG-8)

[oferchen]

Summary

• Audit-only design doc docs/design/wpg-8-send-zc-windows-equivalent.md ratifying TransmitFile as the primary Windows zero-copy socket-send primitive, with RIOSend deferred and TransmitPackets rejected.
• Maps each rsync sender producer (Literal tokens, Copy tokens, multiplex headers, negotiation, compressed streams) to its recommended Win32 API; primary recommendation is TransmitFile for every file-sourced byte.
• Walks the SEND_ZC dual-CQE contract (transfer + notification) and explains why Win32 has no peer for that shape: every Windows candidate posts a single completion, and the buffer-lifetime invariant the second CQE exists to signal is folded into the chosen primitive (file HANDLE owns data, RIO_BUFFERID registration bounds the slab).
• Identifies WPG-8.a through WPG-8.f as follow-up implementation subtasks (async TransmitFile via the existing iocp::pump, lpTransmitBuffers.Head envelope wiring, PlatformSendFile trait, AV warmup probe, CLI flag surface, RIO deferred behind WPG-9).

Inputs and lineage

• WPG-7.a opcode inventory docs/design/wpg-7-iouring-opcode-inventory.md (docs(fast_io): inventory io_uring opcodes (WPG-7.a) #4755): SEND_ZC is feature-gated and floored at 4 KiB.
• WPG-7.b IOCP mapping docs/design/wpg-7b-iouring-iocp-mapping.md (docs(fast_io): map io_uring opcodes to IOCP equivalents (WPG-7.b) #4760): SEND_ZC has a viable IOCP path with a different notification shape.
• WPG-7.c gap list docs/design/wpg-7c-iocp-gap-list.md (docs(fast_io): prioritised IOCP gap list (WPG-7.c) #4765): SEND_ZC is P3, deferred to WPG-8 for the feasibility decision.
• Companion docs already on master: windows-transmitfile.md (Refine cross-platform packaging workflows #2130 API survey), windows-transmitfile-zerocopy.md (Refine cross-platform packaging workflows #2130 integration plan), iouring-send-zc.md (Linux design).
• Existing partial implementation: crates/fast_io/src/iocp/transmit_file.rs synchronous shim plus IocpSocketWriter::try_transmit_file_path fallback wiring.

Test plan

• No source changes; doc-only PR. CI fmt + clippy must remain green.
• CI doc-link / markdown lint passes on the new file.
• Cross-references resolve (WPG-7.a, WPG-7.b, WPG-7.c, companion windows-transmitfile*.md, iouring-send-zc.md).