flow: Phase 1 declarative DAG — sequence/parallel/defflow combinators + 18 tests

Flow combinators as a Scheme prelude loaded onto scheme-standard-env; a flow is a
Scheme procedure input->output, run inside the interpreter (sets up Phase 3 call/cc
suspend). flow/start entry point, conformance runner, scoreboard.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/flow/scoreboard.json

@@ -0,0 +1,11 @@
+{
+  "total": 18,
+  "passed": 18,
+  "failed": 0,
+  "suites": {
+    "basic": { "passed": 18, "total": 18 }
+  },
+  "phases": {
+    "phase1": "in-progress"
+  }
+}