[PIR][WP26] ReCache-pattern content-addressed schema-resource cache + TTFT benchmark (ADR-329)

Part of PIR Wave 4 — epic #837.

Build a content-addressed schema-resource cache in crates/mcp-gate: SHA-256 resource identity bound to resolved content (never self-declared hashes), compile-once reusable blocks, context assembly from cached blocks, downgrade-only hit accounting, and a TTFT-proxy benchmark against a real uncached baseline targeting the >=2x acceptance clause. Pattern reimplementation from arXiv:2608.19662 only — EIT-NLP/ReCache has NO LICENSE file, no code copying. Note: AstraNetLab/CacheRoute is NOT arXiv:2608.19677's code (name collision).

• Evidence grade & artifact availability: docs/research/perpetual-intelligence-runtime/10-wave4-evidence-review.md (PR #892)
• WP/ADR mapping, phases, and security gates: docs/research/perpetual-intelligence-runtime/11-wave4-program-plan.md (PR #892)
• ADR: docs/adr/ADR-329-recache-pattern-schema-resource-cache.md

Depends on: none hard. Security-labeled: new trust surface; run npx @claude-flow/cli@latest security scan after the write path lands.

Read docs/adr/ADR-329-recache-pattern-schema-resource-cache.md together with the Wave 4 evidence review and program plan, then inspect the crates/mcp-gate entry points. Establish the uncached baseline and TTFT-proxy benchmark before changing the write path. Done means the cache behavior and >=2x benchmark acceptance clause are demonstrated, with the security scan run after the write path lands.