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>

lib/artdag/optimize.sx

@@ -4,6 +4,7 @@
 ;            already collapse to one node at build time (artdag/cse == build).
 ;   Fusion — collapse a maximal 1-to-1 chain of fusible unary ops into a single
 ;            "artdag/pipeline" node that replays the stages; output-equivalent.
+;   optimize — fuse then DCE in one pass.
 ; Depends on dag.sx and analyze.sx.
 
 ; ---- dict helper ----
@@ -188,3 +189,14 @@
         (= op artdag/pipeline-op)
         ((artdag/pipeline-run base-runner) params inputs)
         (base-runner op params inputs)))))
+
+; ---- full optimization pass ----
+; fuse the entry list, then drop everything not feeding the requested output
+; names. Output names survive fusion (sinks are never absorbed).
+(define
+  artdag/optimize
+  (fn
+    (entries outputs fusible?)
+    (let
+      ((fused (artdag/fuse entries fusible?)))
+      (artdag/dce fused (map (fn (nm) (artdag/dag-id fused nm)) outputs)))))