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>