docs(design): BR-6 beta-readiness sign-off check-in

[oferchen]

Summary

• BR-6 unblocks now that WPG-1 closed as deferred (docs(design): close WPG-1 as deferred to post-beta Windows hardware capture #4688). This is the sign-off check-in (not the sign-off itself; that lands when SEC-1.i + SEC-1.j merge).
• Enumerates what shipped this session grouped by initiative (SEC-1, IUS, PIP, BR-3j, ABW, FFB, SSC, SSF, RJN, matching engine, WPG, token-loop scaffolding) with PR/commit cites.
• Captures in-flight (SEC-1.i feat(fast_io): fchmodat/fchownat/utimensat sandbox helpers (SEC-1.i) #4690, SEC-1.j) and deferred items (WPG-1, BR-3j.f, ABW-3/4, IUS-4/5/6, PIP-4, RJN-3/4) with per-item rationale for beta acceptability.
• Recommendation: PROCEED with beta tag once SEC-1.i and SEC-1.j land on master; beta release notes should highlight WPG-1 caveat, SEND_ZC opt-in status, and parallel-receive-delta default-on.

Test plan

• Doc renders cleanly on GitHub (markdown sanity)
• Cited PR numbers resolve (docs(design): close WPG-1 as deferred to post-beta Windows hardware capture #4688, docs(security): partial-mitigation status for CVE-2026-29518/43619 (SEC-1.o-partial) #4672, feat(fast_io): fchmodat/fchownat/utimensat sandbox helpers (SEC-1.i) #4690, perf(transfer): enable parallel receive-delta by default via Path B heuristic (PIP-3 + PIP-5) #4666, docs(design): close FFB-3/FFB-4/PIP-2 as satisfied by FFB-1 design + PIP-3+5 wire-up #4677, docs(design): close ABW-3 as N/A pending per-file Mutex refactor #4685, docs(design): defer ABW-2/3/4 pending BR-3j.f bench evidence (ABW-1 audit closure) #4673, docs(audit): evaluate ssh_config parsers for SSC-3 double-compression detection #4674, docs(design): defer RJN-3 (fanout) and RJN-4 (bench) as N/A after RJN-2 rename #4676, docs(design): close RJN-4 as N/A (RJN-3 was rename-only) #4686, docs(design): pre-frame IUS-4 SEND_ZC opt-in vs default-on decision #4687, docs(design): close PIP-4 - interop suite exercises parallel-receive-delta path via PIP-5 default flip #4689, feat(engine): add flush_workers/drain_inflight barrier API to ParallelDeltaApplier (FFB-2) #4665, bench(engine): scaffold PIP-6 end-to-end parallel-vs-sequential bench harness #4679, docs: note --zero-copy SEND_ZC build-time dependency (IUS-1) #4661, docs(audit): IORING_OP_SEND_ZC kernel compatibility matrix (IUS-2) #4664, perf(engine): add bench harness for parallel verify_chunk cores-vs-throughput (BR-3i.f) #4653)
• Cross-reference paths exist (docs/audits/br-4a-workspace-coverage-2026-05-20.md, docs/audits/br-13-beta-bench-2026-05-20.md, docs/design/br-3j-f-dashmap-rebench-2026-05-21.md, .github/RELEASE_NOTES_BETA.md on the BR-7 scaffold branch)
• No references to AI tooling or CLAUDE.md
• Line count under 300 (currently 218)