flow: timeout combinator — cooperative step budget + 7 tests (Phase 2 complete)

(timeout budget node) bounds a node deterministically: nodes opt in via (tick),
budget ticks are allowed, the next raises flow-timeout. No scheduler/clock in pure
SX so the budget is a step count, not wall-clock. Budgets nest and are per-run.

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

lib/flow/scoreboard.json

@@ -1,10 +1,10 @@
 {
-  "total": 42,
-  "passed": 42,
+  "total": 49,
+  "passed": 49,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
-    "control": { "passed": 24, "total": 24 }
+    "control": { "passed": 31, "total": 31 }
   },
-  "phases": { "phase1": "done", "phase2": "in-progress" }
+  "phases": { "phase1": "done", "phase2": "done", "phase3": "pending" }
 }