feat(dag-viewer): stack the stage lanes top to bottom

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Problem

The Stages view put Stage 0, 1 and 2 side by side, so each lane got about a third of the page. Larger DAGs were squeezed small.

Changes

• The three lanes are stacked top to bottom: Stage 0 first, the chosen Stage 1 candidate below it, then the chosen Stage 2 candidate. Each lane is as wide as the page allows.
• The page's maximum width goes from 1,680 px to 1,920 px, and each lane is 560 px tall.
• The Stage 3 list stays beside the lanes and sticks while the page scrolls, with a height that fits the window. Below 820 px it stacks above the lanes as before.
• The README and the app.js header describe the new layout.

CSS and comments only. No change to stages.js or to the document format.

Test plan

• python3 -m unittest test_viewer in tools/dag-viewer (with py-mini-racer): all tests OK, none skipped.
• The published Stage Viewer artifact uses the same page and shows the stacked lanes.

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