feat(dag-viewer): standalone Python render + animated transitions

[zzylol]

Summary

Stacked on #249 (targets that branch, not main, until it merges). Adds a
headless/no-browser rendering path for tools/dag-viewer, plus animated
transitions in the existing page.

What's here

• render.py — bakes a dag_export WorkloadGraph JSON (or several,
  merged the same way index.html's multi-file drop does — colliding query
  names get the source filename appended) into one self-contained HTML
  file: every vendored script and the query data inlined, so the output
  opens directly with no server and no drag-and-drop needed.

  index.html's own fetch('dag.json') convenience is blocked as
  cross-origin by most browsers when opened over file:// — embedding the
  data directly sidesteps that too, not just the no-browser case.
  --mode compare|union opens the page straight into that view with every
  loaded query pre-selected.

  cargo run -p asap-devtools --bin dag_export -- --sql "..." --name q1 \
    | python3 tools/dag-viewer/render.py -o rendered.html --mode union

• viewer.js — index.html's inline <script> extracted verbatim
  into its own file, so index.html and render.py's output share one
  copy of the interaction logic instead of forking it. Pure extraction plus
  the animation/embedded-data additions below — no behavior change to the
  existing single/compare/union element-building logic.

• Animated transitions — mode switches, the shared-subtree ring
  toggling, and initial layout now animate (dagre's animate: true, a
  transition-property on node/edge style so class/style changes ease
  instead of snapping, and a fade-in on newly built elements) instead of
  hard-cutting to a new static layout on every render() call.

• test_render.py — unit tests for render.py's merge/inline/escape
  logic (plain unittest.TestCase, matching tools/clickhouse's
  convention; not wired into CI, same as that tool — this repo doesn't run
  Python in CI today).

• README/RUNNING doc updates for both of the above.

Deliberately out of scope

Query cost. dag_export doesn't emit one today — no cost estimator is
wired into pre-ASAP IR. Nothing renders it because there's nothing to
render; the side panel already dumps a clicked node's detail verbatim, so
a future cost field added there shows up automatically, no viewer change
needed.

Validation

• python3 -m unittest tools/dag-viewer/test_render.py — 10/10 passing.
• render.py's output checked against dag.example.json: JSON
  round-trips byte-for-byte through the embedded <script> tag, esprima
  parses every inlined <script> block, no <script src=...> tags remain.
• The new viewer.js logic (embedded-data loading, --mode override,
  animated-layout config) exercised in a stubbed V8 context via
  py_mini_racer — same validation approach feat(dag-viewer): lane-based Compare/Union DAG-merge view #249's own description used —
  with a capturing cytoscape() stub asserting on the elements/layout
  objects the code hands it (real vendored cytoscape/dagre are unmodified
  and already covered by feat(dag-viewer): lane-based Compare/Union DAG-merge view #249's own validation).
• Manually rendered dag.example.json through render.py --mode union and
  opened the output.

Screenshot

Per review feedback, edge arrows now point from input to consumer
(data-flow direction) instead of parent to child — render.py --mode union output on dag.example.json, top-to-bottom: scans at top, each
query's root at bottom.

Single-query view, same fix:

🤖 Generated with Claude Code

[milindsrivastava1997]

I think the order of arrows should be reversed. Otherwise, lgtm

[zzylol]

@milindsrivastava1997 done — reversed the edge direction in all three view modes (single/compare/union), arrows now point from input to consumer instead of parent to child. Pushed in 16310d4. Screenshot added to the PR description showing the union-mode render with the fix.