flow: branch combinator (conditional) + 6 tests

Phase 2 control flow. (branch pred then else) selects then/else node by running
pred on the threaded input; named 'branch' since 'cond' is a Scheme special form.

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` → **18/18** (Phase 1 in progress)
+`bash lib/flow/conformance.sh` → **24/24** (Phase 1 done; Phase 2 in progress)
 
 ## Ground rules
 
@@ -75,7 +75,8 @@ lib/flow/spec.sx           lib/flow/runtime.sx          lib/flow/store.sx
 
 ## Phase 2 — Control flow + error handling
 
-- [ ] `cond` combinator — predicate selects branch
+- [x] `cond` combinator — predicate selects branch (named `branch`; `cond` is a
+  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