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>

plans/flow-on-sx.md

@@ -16,7 +16,7 @@ federation extension via fed-sx for remote-node execution.
 
 ## Status (rolling)
 
-`bash lib/flow/conformance.sh` → **30/30** (Phase 1 done; Phase 2 in progress)
+`bash lib/flow/conformance.sh` → **36/36** (Phase 1 done; Phase 2 in progress)
 
 ## Ground rules
 
@@ -79,7 +79,8 @@ lib/flow/spec.sx           lib/flow/runtime.sx          lib/flow/store.sx
   Scheme special form). `(branch pred then else)` — 6 tests.
 - [ ] `retry n [backoff]` — re-runs node up to n times on exception
 - [ ] `timeout ms` — bounds node execution
-- [ ] `try-catch` — exception handler with reified error
+- [x] `try-catch` — exception handler with reified error: `(try-catch node handler)`
+  runs node; on raise, calls `(handler error)` and returns its value. 6 tests.
 - [x] error model — exceptions vs explicit `(fail reason)` results: `fail`/`failed?`/
   `fail-reason` produce/inspect failure values that flow downstream as data
   (distinct from raised exceptions caught by retry/try-catch). 6 tests.