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/spec.sx

@@ -7,7 +7,7 @@
 ;; Scheme interpreter so that Phase 3's `suspend` (call/cc) can capture the
 ;; flow continuation directly.
 ;;
-;; Phase 1 combinators:
+;; Phase 1 combinators (flow-combinators-src):
 ;;   (flow-node f)      — wrap a 1-arg procedure as a node (identity)
 ;;   (flow-id input)    — pass the upstream value through unchanged
 ;;   (flow-const v)     — node that ignores input and yields v
@@ -15,15 +15,24 @@
 ;;   (parallel n ...)   — fan input to every child, join results into a list
 ;;                        (SEQUENTIAL evaluation; true concurrency is Phase 3)
 ;;   (defflow name body)— bind a named flow
+;;
+;; Phase 2 combinators (flow-control-src):
+;;   (branch pred then else) — pred on input selects then/else node
+;;     (`cond` is a Scheme special form, so the combinator is named `branch`)
 
 (define
   flow-combinators-src
   "(define (flow-node f) f)\n   (define (flow-id input) input)\n   (define (flow-const v) (lambda (input) v))\n   (define (flow-seq-step ns v)\n     (if (null? ns) v (flow-seq-step (cdr ns) ((car ns) v))))\n   (define sequence (lambda ns (lambda (input) (flow-seq-step ns input))))\n   (define parallel (lambda ns (lambda (input) (map (lambda (n) (n input)) ns))))\n   (define-syntax defflow\n     (syntax-rules ()\n       ((defflow nm body) (define nm body))))")
 
+(define
+  flow-control-src
+  "(define (branch pred then else)\n     (lambda (input) (if (pred input) (then input) (else input))))")
+
 (define
   flow-load-combinators!
   (fn
     (env)
     (begin
       (scheme-eval-program (scheme-parse-all flow-combinators-src) env)
+      (scheme-eval-program (scheme-parse-all flow-control-src) env)
       env)))