artdag: Phase 5 optimization — DCE + CSE + adjacent-op fusion + 18 tests

optimize.sx adds three result-preserving passes: dce (keep outputs + ancestors,
preserve ids), cse (==build; structural sharing is free from content addressing),
and fuse (collapse 1-to-1 fusible unary chains into an artdag/pipeline node fed by
the chain head's input; leaves/fan-out/non-fusible ops never fuse). fusing-runner
replays pipeline stages, output-equivalent to the unfused dag. optimize 18/18,
total 87/87.

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

lib/artdag/scoreboard.json

@@ -3,9 +3,10 @@
     "dag": {"pass": 20, "fail": 0},
     "analyze": {"pass": 16, "fail": 0},
     "plan": {"pass": 18, "fail": 0},
-    "execute": {"pass": 15, "fail": 0}
+    "execute": {"pass": 15, "fail": 0},
+    "optimize": {"pass": 18, "fail": 0}
   },
-  "total_pass": 69,
+  "total_pass": 87,
   "total_fail": 0,
-  "total": 69
+  "total": 87
 }