fix(devtools): clean up Pre/Post-ASAP DAG viewer

[zzylol]

Summary

Stacked on #283. This follow-up keeps #283 unchanged while addressing the DAG-viewer review findings:

• use exporter-assigned workload_node_id for shared-node highlighting
• aggregate explicit post-ASAP decisions from every owner of a merged workload node
• remove obsolete Single/Compare/Union rendering and detail paths
• remove the unused SVG icon system, stale edge-schema selector, and old Before/After naming
• stop Python from preprocessing replacement mini-graphs that the current viewer does not render
• update the viewer contract/docs to describe explicit workload identity

The viewer still resolves target_pre_id and replacement output kind through the explicit decision_id mapping in exported JSON; it does not infer node signatures, strategies, or sharing in the browser.

Validation

• python3 -m unittest discover -s tools/dag-viewer -p 'test_render.py' (18 passed)
• cargo test -p asap-devtools --bin dag_export (6 passed)
• cargo clippy -p asap-devtools --bin dag_export -- -D warnings
• standalone HTML generated from post_asap_fixture.json
• viewer.js/node-style.js syntax parsed (modern optional chaining normalized for the installed parser)

Stack

Base: #283 (feat/dag-post-asap-viz)