docs(test-support): RFC for protocol-level capture/replay harness (#1850)

[oferchen]

Summary

• Adds docs/audits/capture-replay-harness.md (425 lines), an RFC sketch for a protocol-level capture/replay test harness in the test-support crate.
• Surveys current state on master: crates/test-support/src/lib.rs (31 lines, single helper), flat golden_protocol_v28_*.rs / golden_handshakes.rs files in crates/protocol/tests/, and the live tools/ci/run_interop.sh harness.
• Proposes a binary .ocrc capture format with a versioned header and per-frame records (direction, timestamp delta, MessageCode, payload), captured post-decompression at the multiplex frame boundary, gated by OC_RSYNC_CAPTURE_PATH.
• Defines two replay modes (strict byte-equal vs structural) behind a single Replayer type, slotting into existing Read+Write transport seams (crates/rsync_io/src/ssh/connection.rs, daemon negotiation path).
• Cross-references existing fuzz tasks (Add support for --no-bwlimit #1304, Refactor branding tests into dedicated module #1365, Prevent module burst from raising daemon bwlimit #1196, Handle truthy fallback overrides as default delegation #1303), completed pattern reference (Add helpers to convert BandwidthLimit into a limiter #1191), and internal docs's Strategy and Dependency Inversion guidance.
• Phasing: this RFC is Phase 1; phases 2-5 deliver Recorder, strict-mode Replayer with first regression test, structural mode, and optional fuzz seed-corpus integration.

Includes Last verified: 2026-05-01 marker and explicit "RFC, sketch only. No implementation in this PR." at the top.

Test plan

• All cited oc-rsync paths and line numbers verified against master via Read/Grep before authoring.
• Document length: 425 lines (within target 300-450).
• Hyphens only - no em-dashes or en-dashes (grep confirmed clean).
• No Rust code added; docs-only PR.
• CI: docs-only change; expect fmt+clippy, nextest, Windows/macOS/Linux musl checks all pass without rebuild.