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/api.sx

@@ -0,0 +1,30 @@
+;; lib/flow/api.sx — flow runtime entry points.
+;;
+;; Builds a Scheme env preloaded with the flow combinators (lib/flow/spec.sx)
+;; plus the public flow API, and provides SX helpers to run flow programs.
+;;
+;; Scheme-level API (available inside flow programs):
+;;   (flow/start flow input) — run a flow with the given input, return result
+;;
+;; SX-level helpers (for hosts and tests):
+;;   (flow-make-env)        — fresh standard env + combinators + api
+;;   (flow-run src)         — eval a Scheme program string in a fresh flow env
+;;   (flow-run-in env src)  — eval a Scheme program string in a given env
+
+(define flow-api-src "(define (flow/start flow input) (flow input))")
+
+(define
+  flow-make-env
+  (fn
+    ()
+    (let
+      ((env (scheme-standard-env)))
+      (flow-load-combinators! env)
+      (scheme-eval-program (scheme-parse-all flow-api-src) env)
+      env)))
+
+(define
+  flow-run-in
+  (fn (env src) (scheme-eval-program (scheme-parse-all src) env)))
+
+(define flow-run (fn (src) (flow-run-in (flow-make-env) src)))