feat(bench): add --out summary CSV flag + verify v0.5.35 CLI hot-path baseline

[githubrobbi]

Standalone v0.6.0 release-readiness work, surfaced during the 2026-05-15 baseline-bench pass on the Windows 7-drive reference box.

Note on rebase. This PR originally also carried the Phase 6 24-h soak closure commit, but that landed independently on main as PR #246 while this branch was being prepared. Rebasing origin/main auto-detected the duplicate patch and dropped it; only the bench commit remains. See PR #246 for the Phase 6 soak closure details.

Plan §1 goal-4 (no CLI hot-path regression) — VERIFIED

Captured 2026-05-15 on the 7-drive reference box (drive D,
7.07 M records, 30 rounds, HOT phase, p50/p95 wall_ms):

Pattern	v0.5.35	v0.5.96	Δ p50	Rows
exact	20 / 23 ms	18 / 19 ms	−10 %	3
prefix	46 / 50 ms	40 / 46 ms	−13 %	8 732
ext_rare	18 / 20 ms	17 / 18 ms	−6 %	11
ext_dll	94 / 100 ms	35 / 40 ms	−63 % (2.7×)	44 529
substring	50 / 54 ms	43 / 47 ms	−14 %	12 458

Current v0.5.96 is universally faster than v0.5.35 on every
pattern. Row-count parity preserved exactly across both versions
(no result-set drift). Capture: LOG/memory output (manual
stdout paste — the bench script's --out flag was not yet
available for this run).

New bench-script feature: --out <path>

scripts/windows/cross-tool-benchmark.rs learned a --out <path>
flag that writes the post-run summary as plain CSV (12 columns:
tool,phase,sink,drive,pattern,p50_ms,p95_ms,rows,bad,verdict, rounds_ok,rounds_total). Intermediate directories are created
on demand; the writer exits 2 on failure. Distinct from the
existing <cwd>/uffs_bench_out.csv daemon-side file (raw
per-query row dumps). Banner block now prints both paths up
front so operators see exactly where each artefact lands.

Unknown CLI flags now warn on stderr (warning: unknown flag "--foo" ignored) instead of silently swallowing — caught during
the 2026-05-15 baseline bench where a typo---out in
pre-flag-support runs left the operator hunting a non-existent
CSV file.

v0.6.0 bake gate status (with this PR)

Gate	Status	Closed
Phase 5 (ws-trace) 24-h soak	🟢 4 / 4 PASS	2026-05-13
Phase 7 (USN-journal churn) 24-h soak	🟢 7 / 7 PASS	2026-05-13
Phase 6 (adaptive TTL) 24-h soak	🟢 9 / 9 PASS (#246)	2026-05-15
Plan §1 goal-4 (CLI hot-path no-regression)	🟢 5/5 patterns improved	2026-05-15

All v0.6.0 readiness gates green. Only the one-week main bake
remains per
memory-tiering-bake-criteria.md.

Verification

• just lint-pre-push — 23/23 gates green (54s pre-commit; 50s
  pre-push initial; 3s pre-push post-rebase — bucket 2 skipped
  since no rust/dep/infra files changed).
• Bench script compiles cleanly via standalone rustc --edition 2021 --emit=metadata on Mac (frontmatter intact, no warnings).
• No new code paths in production crates — this is documentation
  plus a Windows-only rust-script.

Files touched

• CHANGELOG.md — two new subsections under v0.6.0 staging:
  ### Verified — v0.5.35 baseline CLI hot-path bench (2026-05-15)
  and ### Added — cross-tool benchmark script: --out summary CSV flag.
• scripts/windows/cross-tool-benchmark.rs — --out flag
  plumbing + write_summary_csv impl (~80 LOC of new code) +
  unknown-flag warning + banner-block restructuring.