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/scoreboard.json

@@ -4,12 +4,12 @@
     "analyze": {"pass": 16, "fail": 0},
     "plan": {"pass": 18, "fail": 0},
     "execute": {"pass": 15, "fail": 0},
-    "optimize": {"pass": 18, "fail": 0},
+    "optimize": {"pass": 22, "fail": 0},
     "fed": {"pass": 15, "fail": 0},
     "cost": {"pass": 13, "fail": 0},
     "serialize": {"pass": 13, "fail": 0}
   },
-  "total_pass": 128,
+  "total_pass": 132,
   "total_fail": 0,
-  "total": 128
+  "total": 132
 }