Add cascade resolver bench harness, p95 gate < 2ms (Phase 1, 6/6)

Builds a 15200-node fixture (50×100 wall grid + 8000 hosted doors +
200 sparsely-indexed slabs) and runs `cascadeDirty` 1000 times with
warm-up. Reports p50/p95/p99/mean/max in ms.

Runs via `bun run bench:registry` from the core package.

Today: p95 measured at ~0.002ms — three orders of magnitude under the
Phase 1 gate of 2ms. Headroom is substantial; we'll only revisit this
if Phase 3 wall introduces `linkedBy: 'endpoint-match'` and pushes the
inner cascade past the gate.

Not wired into CI for v1 — regressions reviewed manually before phase
gates. Output is JSON so a future CI step can diff against a baseline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/package.json

@@ -60,6 +60,7 @@
     "build": "tsc --build",
     "dev": "tsc --build --watch",
     "test": "bun test",
+    "bench:registry": "bun run src/registry/__bench__/relations-resolver.bench.ts",
     "prepublishOnly": "npm run build"
   },
   "peerDependencies": {