artdag: Phase 1 DAG model + structural content addressing + 20 tests

Content-addressed node = {:op :inputs :params :commutative}; content-id is a
deterministic canonical serialization (sorted param keys; commutative ops sort
inputs). artdag/build validates dangling/cycles, topo-sorts, dedups identical
subgraphs to one id shared across DAGs. conformance.sh + scoreboard (dag 20/20).

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

plans/artdag-on-sx.md

@@ -30,7 +30,7 @@ edges.
 
 ## Status (rolling)
 
-`bash lib/artdag/conformance.sh` → **0/0** (not yet started)
+`bash lib/artdag/conformance.sh` → **20/20** (1 suite: dag)
 
 ## Ground rules
 
@@ -78,13 +78,13 @@ lib/artdag/optimize.sx                  lib/artdag/federation.sx
 
 ## Phase 1 — DAG model + content addressing
 
-- [ ] `lib/artdag/dag.sx` — node `{:op :inputs :params}`; structural content-id =
+- [x] `lib/artdag/dag.sx` — node `{:op :inputs :params}`; structural content-id =
   digest of `(op, sorted input-ids, params)`; build/validate a DAG (no dangling
   inputs, no accidental cycles); topological order
-- [ ] identical-subgraph sharing: two structurally-equal nodes get the same id
-- [ ] `lib/artdag/tests/dag.sx` — id determinism, subgraph sharing, cycle/dangling
+- [x] identical-subgraph sharing: two structurally-equal nodes get the same id
+- [x] `lib/artdag/tests/dag.sx` — id determinism, subgraph sharing, cycle/dangling
   rejection, topo order
-- [ ] `lib/artdag/conformance.sh` + scoreboard
+- [x] `lib/artdag/conformance.sh` + scoreboard
 
 ## Phase 2 — Analyze (Datalog)
 
@@ -136,8 +136,19 @@ lib/artdag/optimize.sx                  lib/artdag/federation.sx
 
 ## Progress log
 
-(loop fills this in)
+- **Phase 1 — DAG model + content addressing** (dag suite 20/20). `lib/artdag/dag.sx`:
+  node `{:op :inputs :params :commutative}`; `artdag/content-id` = `"node:"` + a
+  deterministic canonical serialization of `(op, inputs, params)` with dict keys
+  sorted (param order-insensitive) and commutative ops' inputs sorted (input
+  order-insensitive); non-commutative inputs ordered. `artdag/build` takes named
+  entries `(name op (input-names) params [commutative?])`, validates (dangling refs,
+  cycles via fixpoint topo), resolves input-names→content-ids, dedups identical
+  subgraphs to one node + one id (shared across DAGs), returns `{:ok :nodes :names
+  :order}`. No host `sort`/`string<?` — hand-rolled `artdag/str<?` over char-codes.
+  Gotcha logged: SX `equal?` is representation-sensitive (cons-built vs vector lists
+  compare unequal even when identical); `=` is true structural equality — conformance
+  harness compares with `=`. `lib/artdag/conformance.sh` + scoreboard.
 
 ## Blockers
 
-(loop fills this in)
+(none)