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>

lib/flow/conformance.sh

@@ -22,6 +22,7 @@ VERBOSE="${1:-}"
 # Suites: NAME RUNNER-FN PATH
 SUITES=(
   "basic       flow-basic-tests-run!     lib/flow/tests/basic.sx"
+  "control     flow-ctl-tests-run!       lib/flow/tests/control.sx"
 )
 
 TMPFILE=$(mktemp); trap "rm -f $TMPFILE" EXIT