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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(dag analyze plan execute)
+SUITES=(dag analyze plan execute optimize)
 
 OUT_JSON="lib/artdag/scoreboard.json"
 OUT_MD="lib/artdag/scoreboard.md"
@@ -45,6 +45,7 @@ run_suite() {
 (load "lib/artdag/analyze.sx")
 (load "lib/artdag/plan.sx")
 (load "lib/artdag/execute.sx")
+(load "lib/artdag/optimize.sx")
 (epoch 2)
 (eval "(define artdag-test-pass 0)")
 (eval "(define artdag-test-fail 0)")