flow: try-catch combinator — reify raised exceptions + 6 tests

(try-catch node handler) runs node; on a raised exception calls (handler error)
with the reified error via Scheme guard, returns the handler value.

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

lib/flow/scoreboard.json

@@ -1,10 +1,10 @@
 {
-  "total": 30,
-  "passed": 30,
+  "total": 36,
+  "passed": 36,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
-    "control": { "passed": 12, "total": 12 }
+    "control": { "passed": 18, "total": 18 }
   },
   "phases": { "phase1": "done", "phase2": "in-progress" }
 }