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.md

@@ -1,6 +1,6 @@
 # flow-on-sx Scoreboard
 
-**All tests pass: 24 / 24 across 2 suites.**
+**All tests pass: 30 / 30 across 2 suites.**
 
 `bash lib/flow/conformance.sh`
 
@@ -9,7 +9,7 @@
 | Suite | Passing | Covers |
 |-------|--------:|--------|
 | basic |     18  | Phase 1: single nodes, linear sequence, data-flow threading, defflow, parallel fan/join, nested composition, publish-shaped flow |
-| control |   6  | Phase 2: `branch` conditional — true/false select, threaded predicate, full-node branches, nested 3-way, approval gate |
+| control |  12  | Phase 2: `branch` conditional (6); error model `fail`/`failed?`/`fail-reason` failure values that flow as data (6) |
 
 ## Architecture
 
@@ -36,6 +36,6 @@ capture the flow continuation directly.
 ## Phases
 
 - [x] Phase 1 — Declarative DAG + sequential execution (combinators + 18 tests, `flow/start`)
-- [~] Phase 2 — Control flow + error handling (`branch` done)
+- [~] Phase 2 — Control flow + error handling (`branch`, error model done)
 - [ ] Phase 3 — Suspend / resume (the showcase)
 - [ ] Phase 4 — Distributed nodes via fed-sx