flow: error model — fail/failed?/fail-reason failure values + 6 tests

Explicit (fail reason) values flow downstream as data and are inspected with
failed?/fail-reason — distinct from raised exceptions (retry/try-catch territory).

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

lib/flow/scoreboard.json

@@ -1,13 +1,10 @@
 {
-  "total": 24,
-  "passed": 24,
+  "total": 30,
+  "passed": 30,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
-    "control": { "passed": 6, "total": 6 }
+    "control": { "passed": 12, "total": 12 }
   },
-  "phases": {
-    "phase1": "done",
-    "phase2": "in-progress"
-  }
+  "phases": { "phase1": "done", "phase2": "in-progress" }
 }