flow: bounded iteration combinators flow-while/flow-until + 6 tests

(flow-while pred body max) / (flow-until pred body max) re-run body threading the
value while/until pred holds, capped at max steps for a deterministic bound (no
unbounded loops in pure SX). 122/122 across 7 suites.

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

lib/flow/scoreboard.json

@@ -1,6 +1,6 @@
 {
-  "total": 116,
-  "passed": 116,
+  "total": 122,
+  "passed": 122,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
@@ -9,7 +9,7 @@
     "recovery": { "passed": 8, "total": 8 },
     "distributed": { "passed": 19, "total": 19 },
     "api": { "passed": 12, "total": 12 },
-    "combinators": { "passed": 11, "total": 11 }
+    "combinators": { "passed": 17, "total": 17 }
   },
   "phases": { "phase1": "done", "phase2": "done", "phase3": "done", "phase4": "done", "phase5": "done" }
 }