artdag: optimize composition pass (fuse + dce) + 4 tests

artdag/optimize entries outputs fusible? fuses the entry list then DCEs against
the output names — sinks survive fusion (never absorbed), so output-equivalent
with fewer nodes. optimize 22/22, total 132/132.

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` → **128/128** (8 suites: dag, analyze, plan, execute, optimize, fed, cost, serialize)
+`bash lib/artdag/conformance.sh` → **132/132** (8 suites: dag, analyze, plan, execute, optimize, fed, cost, serialize)
 
 Base roadmap (Phases 1–6) COMPLETE. Now extending.
 
@@ -138,6 +138,12 @@ lib/artdag/optimize.sx                  lib/artdag/federation.sx
 
 ## Progress log
 
+- **Ext: optimize composition pass** (optimize suite 22/22, total 132/132).
+  `artdag/optimize entries outputs fusible?` fuses the entry list then DCEs against
+  the output names (sinks survive fusion since they're never absorbed) — fewer nodes,
+  identical results. Verified: dead branch dropped + chain fused (4→2), an output that
+  is itself "dead" is retained, no-fusible-set still DCEs.
+
 - **Ext: DAG wire serialization** (serialize suite 13/13, total 128/128).
   `lib/artdag/serialize.sx`: `dag->wire` emits a topo-ordered list of
   `(id op inputs params commutative)` records — plain lists with keyword-keyed param