feat(summary): revision-gated cache with stale-while-revalidate (STATE-WP-0066)

Replace the fixed 15s TTL on GET /state/summary with per-table revision
watermarks, stale-while-revalidate background refresh, and a progress-tail
section split. SQLAlchemy write hooks invalidate core or progress sections
on mutation. Adds tests, benchmark script, and operator docs.

scripts/benchmark_summary_cache.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+"""Quick benchmark for /state/summary revision cache (STATE-WP-0066)."""
+from __future__ import annotations
+
+import argparse
+import statistics
+import sys
+import time
+import urllib.request
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--base-url", default="http://127.0.0.1:8000")
+    parser.add_argument("--requests", type=int, default=10)
+    args = parser.parse_args()
+
+    url = f"{args.base_url.rstrip('/')}/state/summary"
+    timings: list[float] = []
+    last_cache = ""
+
+    # Prime cache
+    with urllib.request.urlopen(url, timeout=30) as resp:
+        resp.read()
+        last_cache = resp.headers.get("X-StateHub-Cache", "")
+
+    for _ in range(args.requests):
+        started = time.perf_counter()
+        with urllib.request.urlopen(url, timeout=30) as resp:
+            resp.read()
+            last_cache = resp.headers.get("X-StateHub-Cache", "")
+        timings.append(time.perf_counter() - started)
+
+    p95 = statistics.quantiles(timings, n=20)[18] if len(timings) >= 2 else timings[0]
+    print(f"requests={args.requests} p50={statistics.median(timings):.3f}s p95={p95:.3f}s last_cache={last_cache}")
+    return 0 if last_cache == "hit-revision" else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())