fix: v0.21.1 — harden the fast-path gate

- Analysis lane exempt from the Codex health gate (no Codex session needed)
- Mechanical scope trip-wire: extra changed files auto-escalate to full flow
- Small-change lane gets its own known-red test baseline
- Durable fastpath.log preserves session lineage and audit trail
- Measure input sizes (du -h) before applying the ~50MB data-tooling rule
- Guard tests lock the fast-path contract and data tooling rules