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/scoreboard.json

@@ -1,11 +1,13 @@
 {
-  "total": 18,
-  "passed": 18,
+  "total": 24,
+  "passed": 24,
   "failed": 0,
   "suites": {
-    "basic": { "passed": 18, "total": 18 }
+    "basic": { "passed": 18, "total": 18 },
+    "control": { "passed": 6, "total": 6 }
   },
   "phases": {
-    "phase1": "in-progress"
+    "phase1": "done",
+    "phase2": "in-progress"
   }
 }