flow: store hygiene flow/gc + flow/forget + 9 tests

flow/gc drops terminal (done/cancelled) records, keeps live suspended flows, returns
count removed; flow/forget id drops one terminal record and refuses live flows.
Bounds unbounded store growth (retention/GC). Bumped conformance sx_server timeout
to 540s for the 10-suite run under CPU contention. 151/151 across 10 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/flow/scoreboard.json

@@ -1,6 +1,6 @@
 {
-  "total": 142,
-  "passed": 142,
+  "total": 151,
+  "passed": 151,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
@@ -11,7 +11,8 @@
     "api": { "passed": 12, "total": 12 },
     "combinators": { "passed": 17, "total": 17 },
     "railway": { "passed": 10, "total": 10 },
-    "integration": { "passed": 10, "total": 10 }
+    "integration": { "passed": 10, "total": 10 },
+    "hygiene": { "passed": 9, "total": 9 }
   },
   "phases": { "phase1": "done", "phase2": "done", "phase3": "done", "phase4": "done", "phase5": "done", "phase6": "done", "phase7": "done" }
 }