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>

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` → **24/24** (Phase 1 done; Phase 2 in progress)
+`bash lib/flow/conformance.sh` → **30/30** (Phase 1 done; Phase 2 in progress)
 
 ## Ground rules
 
@@ -80,7 +80,9 @@ lib/flow/spec.sx           lib/flow/runtime.sx          lib/flow/store.sx
 - [ ] `retry n [backoff]` — re-runs node up to n times on exception
 - [ ] `timeout ms` — bounds node execution
 - [ ] `try-catch` — exception handler with reified error
-- [ ] error model — exceptions vs explicit `(fail :reason ...)` results
+- [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.
 - [ ] `lib/flow/tests/control.sx` — 25+ cases: each combinator + composition
 
 ## Phase 3 — Suspend / resume (the showcase)