docs(fast_io): audit kqueue cross-platform gates (XPL-2)

[oferchen]

Summary

• Audits the kqueue surface in fast_io (crates/fast_io/src/kqueue/mod.rs, crates/fast_io/src/kqueue_stub.rs, and the lib.rs mount + re-export site) against the project Cross-Platform conventions.
• All eight items audited are already correctly gated: 0 CI-fatal / 0 Warning / 8 Clean. No code changes required.
• Companion to XPL-1 (apple-fs crate, chore(apple-fs): audit + fix cross-platform compile hazards (XPL-1) #4743); same audit pattern extended to the next platform-conditional surface.

Findings (clean)

1. lib.rs mount switch is symmetric and exhaustive (target_os = "macos" vs not(target_os = "macos")).
2. Stub file-level #![cfg(not(target_os = "macos"))] + #![allow(dead_code)] are correct.
3. RawFd alias compiles on every target without referencing libc (which is cfg(unix)-only in Cargo.toml).
4. KEventFilter variants are not platform-dependent; no "gate the variant, not the impl" hazard.
5. KEvent struct fields are pub; no platform-dependent dead-field hazard.
6. unsafe impl Send for KqueueLoop {} lives in the macOS-only module, never compiled on Windows or musl.
7. Test modules are correctly gated via their parent (no orphan #[cfg(unix)] tests inside ungated modules).
8. Re-export in lib.rs:282 uses backtick-only prose, dodging the rustdoc re-export link breakage.

See docs/design/xpl-2-kqueue-audit.md for the full per-item walk plus methodology.

Test plan

• CI: fmt+clippy passes (doc-only change).
• CI: nextest (stable) passes.
• CI: Windows (stable) passes.
• CI: macOS (stable) passes.
• CI: Linux musl (stable) passes.