feat: pgGraph-inspired CI scaffolding wave (v0.37.4.0) (#1228)

Schema-migration matrix + fuzz harness + RSS budget gate + read-latency
under sync + sync lock regression + tests/heavy convention + nightly CI
workflow + BFS frontier cap on traverseGraph.

CI infra (T1-T7):
- tests/heavy/ directory convention + scripts/run-heavy.sh + bun run test:heavy
- tests/heavy/pg_upgrade_matrix.sh: walk pre-v0.13 + pre-v0.18 brain shapes
  forward to head via bootstrap → SCHEMA_SQL → migrations → verifySchema
- test/fuzz/{pure,mixed,filesystem}-validators.test.ts: 1000-run fast-check
  property tests across 8 trust-boundary validators
- scripts/check-fuzz-purity.sh: bun-bundle + grep guard, wired into verify
- tests/heavy/measure_rss.sh: in-memory PGLite workload + peak RSS measurement
  via /proc/self/status (Linux) or process.memoryUsage().rss fallback (macOS,
  refuses to write baseline)
- tests/heavy/read_latency_under_sync.sh: phase A baseline + phase B under
  parallel writer load, reports p50/p95/p99 + delta_pct
- tests/heavy/sync_lock_regression.sh: N concurrent gbrain sync against one
  DB, asserts 1 winner + N-1 lock-busy + zero leaked gbrain_cycle_locks rows
- .github/workflows/heavy-tests.yml: cron '17 8 * * *' + heavy-tests label
  trigger + Postgres service + artifact upload on failure

Engine (T8):
- BrainEngine.traverseGraph opts gain frontierCap?: number + onTruncation?:
  (info: TruncationInfo) => void callback. Return shape preserved
  (Promise<GraphNode[]>) for MCP wire stability.
- Postgres CTE: parenthesized LIMIT N ORDER BY (slug, id) inside recursive term.
- PGLite: same SQL with positional params.
- Per-call callback closure — not engine-instance state — so concurrent
  traversals on the same engine don't cross-talk. 5 contracts pinned in
  test/regressions/v0_36_frontier_cap.test.ts.

Three plan-review passes ran before any code: CEO scope review (Approach C),
Eng dual-voice review (Claude subagent + Codex), and Codex 2nd-pass against
the revised plan. The 2nd pass caught issues the first two missed (Bun ESM
vs require.cache; engine-instance metadata stomping under concurrency;
fixture-size inconsistency). All addressed.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

bun.lock

@@ -39,6 +39,7 @@
         "@types/cors": "^2.8.19",
         "@types/express": "^5.0.6",
         "bun-types": "^1.3.13",
+        "fast-check": "^4.8.0",
         "typescript": "^5.6.0",
       },
     },
@@ -373,6 +374,8 @@
 
     "extend-shallow": ["extend-shallow@2.0.1", "", { "dependencies": { "is-extendable": "^0.1.0" } }, "sha512-zCnTtlxNoAiDc3gqY2aYAWFx7XWWiasuF2K8Me5WbN8otHKTUKBwjPtNpRs/rbUZm7KxWAaNj7P1a/p52GbVug=="],
 
+    "fast-check": ["fast-check@4.8.0", "", { "dependencies": { "pure-rand": "^8.0.0" } }, "sha512-GOJ158CUMnN6cSahsv4+ExARvIDuzzinFjkp0E9WtiBa5zcVeLozVkWaE4IzFcc+Y48Wp1EDlUZsXRyAztQcSg=="],
+
     "fast-deep-equal": ["fast-deep-equal@3.1.3", "", {}, "sha512-f3qQ9oQy9j2AhBe/H9VC91wLmKBCCU/gDOnKNAYG5hswO7BLKj09Hc5HYNz9cGI++xlpDCIgDaitVs03ATR84Q=="],
 
     "fast-uri": ["fast-uri@3.1.0", "", {}, "sha512-iPeeDKJSWf4IEOasVVrknXpaBV0IApz/gp7S2bb7Z4Lljbl2MGJRqInZiUrQwV16cpzw/D3S5j5Julj/gT52AA=="],
@@ -491,6 +494,8 @@
 
     "proxy-addr": ["proxy-addr@2.0.7", "", { "dependencies": { "forwarded": "0.2.0", "ipaddr.js": "1.9.1" } }, "sha512-llQsMLSUDUPT44jdrU/O37qlnifitDP+ZwrmmZcoSKyLKvtZxpyV0n2/bD/N4tBAAZ/gJEdZU7KMraoK1+XYAg=="],
 
+    "pure-rand": ["pure-rand@8.4.0", "", {}, "sha512-IoM8YF/jY0hiugFo/wOWqfmarlE6J0wc6fDK1PhftMk7MGhVZl88sZimmqBBFomLOCSmcCCpsfj7wXASCpvK9A=="],
+
     "qs": ["qs@6.15.0", "", { "dependencies": { "side-channel": "^1.1.0" } }, "sha512-mAZTtNCeetKMH+pSjrb76NAM8V9a05I9aBZOHztWy/UqcJdQYNsf59vrRKWnojAT9Y+GbIvoTBC++CPHqpDBhQ=="],
 
     "range-parser": ["range-parser@1.2.1", "", {}, "sha512-Hrgsx+orqoygnmhFbKaHE6c296J+HTAQXoxEF6gNupROmmGJRoyzfG3ccAveqCBrwr/2yxQ5BVd/GTl5agOwSg=="],