feat(M3.4): Implement composition gate for M3 (L2 + rerank + query)

Adds gate verification that M3.1 (L2 synthesis) + M3.2 (rerank) + M3.3 (query) work together:

Files added:
  verify/known-answers.yaml
    - 3 known-answer questions from real infrastructure findings
    - Expected node texts and source substrings
    - Gate thresholds: hit_rate ≥ 0.8, precision ≥ 0.9

  verify/m3.4.sh (executable)
    - Runs known-answer questions through mem query
    - Measures hit rate at k=5
    - Verifies provenance precision (90%+ of citations contain facts)
    - Checks mem verify for level consistency
    - Checks L2→L1→L0 edge resolution
    - Exit 0 if all thresholds met, 1 if any fail

  tests/it_m3_gate.rs
    - 8 integration tests, 6 marked #[ignore] (need live DB)
    - a1-a2: Known-answer Kong buffer / auth header
    - a3: L2→L1→L0 two-hop provenance walks
    - a4: Reranking improves order
    - a5: No cross-project leakage
    - a6: Level consistency check
    - a7: Query command exists (✅ passes)
    - a8: Verify command works (✅ passes)

Gate criteria (M3 passes when):
  - Hit rate at k=5 ≥ 0.8
  - Provenance precision ≥ 0.9
  - mem verify clean
  - L2→L1→L0 edges resolve
  - Reranking maintains/improves accuracy

Status:
  ✅ Tests compile
  ✅ Smoke tests pass (a7, a8)
  ⏳ Full gate ready for seeded database

Blocks: M4 (skills implementation)
Depends: M3.1 ✅, M3.2 ✅, M3.3 ✅

Cargo.lock

@@ -2476,6 +2476,7 @@ dependencies = [
  "mem-llm",
  "mem-store",
  "serde_json",
+ "sqlx",
  "time",
  "tokio",
  "toml",