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>

plans/flow-on-sx.md

@@ -16,7 +16,7 @@ federation extension via fed-sx for remote-node execution.
 
 ## Status (rolling)
 
-`bash lib/flow/conformance.sh` → **0/0** (not yet started)
+`bash lib/flow/conformance.sh` → **18/18** (Phase 1 in progress)
 
 ## Ground rules
 
@@ -62,15 +62,16 @@ lib/flow/spec.sx           lib/flow/runtime.sx          lib/flow/store.sx
 
 ## Phase 1 — Declarative DAG + sequential execution
 
-- [ ] `lib/flow/spec.sx` — `defflow` macro, `sequence` combinator
-- [ ] node = Scheme thunk; output threads to next node (data flow)
-- [ ] `parallel` combinator (sequential semantics for now — TRUE parallelism in Phase 3)
-- [ ] runtime executes a flow synchronously, returns final value
-- [ ] `lib/flow/api.sx` — `(flow/start name args)` entry point
-- [ ] `lib/flow/tests/basic.sx` — 15+ cases: linear sequence, nested sequences,
-  data flow between nodes, parallel-with-join
-- [ ] `lib/flow/scoreboard.{json,md}`
-- [ ] `lib/flow/conformance.sh`
+- [x] `lib/flow/spec.sx` — `defflow` macro, `sequence` combinator
+- [x] node = Scheme procedure of one arg (upstream value threaded in); output
+  threads to next node (data flow). A node ignoring its arg is a thunk.
+- [x] `parallel` combinator (sequential semantics for now — TRUE parallelism in Phase 3)
+- [x] runtime executes a flow synchronously, returns final value
+- [x] `lib/flow/api.sx` — `(flow/start flow input)` entry point
+- [x] `lib/flow/tests/basic.sx` — 18 cases: single nodes, linear/nested sequence,
+  data flow between nodes, parallel-with-join, publish-shaped flow
+- [x] `lib/flow/scoreboard.{json,md}`
+- [x] `lib/flow/conformance.sh`
 
 ## Phase 2 — Control flow + error handling
 
@@ -101,8 +102,16 @@ lib/flow/spec.sx           lib/flow/runtime.sx          lib/flow/store.sx
 
 ## Progress log
 
-(loop fills this in)
+- **Phase 1 (combinators + sequential runtime).** Flow built as a Scheme prelude
+  loaded onto `scheme-standard-env`: a flow is a Scheme procedure `input -> output`,
+  so the whole flow runs inside the interpreter (sets up Phase 3 call/cc suspend).
+  Combinators `flow-node`/`flow-id`/`flow-const`/`sequence`/`parallel`/`defflow` in
+  `spec.sx`; `flow/start` + SX helpers (`flow-make-env`/`flow-run`) in `api.sx`.
+  18/18 in `tests/basic.sx`. Substrate constraints found: dotted rest params
+  `(a . rest)` and named `let` are unsupported in `lib/scheme/eval.sx`, so
+  combinators use `(lambda args ...)` variadics + top-level recursion. Scheme
+  strings come back boxed as `{:scm-string "..."}` — unwrap with `(get s :scm-string)`.
 
 ## Blockers
 
-(loop fills this in)
+(none)