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/spec.sx

@@ -0,0 +1,29 @@
+;; lib/flow/spec.sx — flow combinators as a Scheme prelude.
+;;
+;; A flow is a Scheme procedure of one argument: the upstream value.
+;;   node : input -> output
+;; A leaf node ignoring its argument is effectively a thunk. Combinators
+;; build composite nodes out of child nodes. The whole flow runs INSIDE the
+;; Scheme interpreter so that Phase 3's `suspend` (call/cc) can capture the
+;; flow continuation directly.
+;;
+;; Phase 1 combinators:
+;;   (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
+;;   (sequence n ...)   — thread input left-to-right through children
+;;   (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
+
+(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-load-combinators!
+  (fn
+    (env)
+    (begin
+      (scheme-eval-program (scheme-parse-all flow-combinators-src) env)
+      env)))