docs(design): IORING_OP_SEND_ZC for network zero-copy (#1832)

[oferchen]

Summary

• Adds docs/design/iouring-send-zc.md (264 lines) covering how the io_uring socket writer at crates/fast_io/src/io_uring/socket_writer.rs will graduate from IORING_OP_SEND to IORING_OP_SEND_ZC on kernel >= 6.0.
• Specifies the two-CQE completion model (notification + release), the pin-counted buffer pool that satisfies the user-page lifetime contract, kernel probe via IORING_REGISTER_PROBE with IORING_OP_SEND fallback, daemon TCP integration via Ensure daemon fallback obeys disable overrides #1876, and the throughput vs latency threshold rule (16 KiB default).
• Cites task Add regression tests for invalid bandwidth limit formats #1832; companion to the fixed-buffer audit and the daemon TPC benchmark plan.

Test plan

• Doc renders in GitHub markdown without heading or anchor breakage.
• Cited line refs in crates/fast_io/src/io_uring/socket_writer.rs, batching.rs, and config.rs resolve to the described code.
• No references to internal tooling, AI provenance, or em-dashes.
• Implementation PRs (probe + pin-counted pool + daemon wiring) link back to this doc.