feat(viewer): make intent-trace-flow animation loop continuously

[sanpatricky]

Problem and value

When meta.animation: "trace" is enabled and the viewer is in Live mode, hovering over a node triggers the intent-trace-flow animation on connected edges. The animation plays once (1.15s) with a fade-in/fade-out (opacity 0.28 → 0.94 → 0), then disappears. Users expect continuous flow while hovering — the single play requires repeatedly moving the mouse to re-trigger, making it hard to observe data flow direction and connectivity.

Fixes #337

Scope

• What changed:
  • archify/assets/template.html line 4417: animation-iteration-count from 1 both to infinite — loops continuously while trace is active.
  • archify/assets/template.html lines 4747–4751: simplified @keyframes archify-intent-trace-flow to maintain constant opacity: 0.94 throughout (removed fade-in from 0.28 and fade-out to 0).
• What deliberately did not change:
  • All existing motion guards preserved: data-motion="still", data-document-hidden="true", data-motion-capable, reduced-motion preference, and export states still set animation: none !important.
  • Other animation keyframes (story-trail-flow, route-probe-flow, route-journey-flow, semantic-lens-flow) are untouched.
  • No schema, validator, or renderer logic changes.
• No unrelated changes: confirmed.

Stability impact

• Compatibility and migration risk: None. Only affects Live mode behavior when meta.animation: "trace" is explicitly authored. Still mode (default), reduced motion, export, and print are unaffected.
• Renderer, validator, package, or generated-artifact risk: Low. CSS-only change in template. Validator does not inspect animation CSS. Generated artifacts from deliver will include the new animation behavior.
• Failure behavior and rollback path: Revert the single commit. Previously delivered HTML is unaffected (self-contained).

Tests run

Not applicable for CSS animation behavior — no existing test exercises animation iteration count. Manual verification: opened a workflow diagram with animation: "trace" in Chrome, switched to Live mode, hovered over nodes, confirmed continuous flow animation that stops correctly when switching to Still mode.

Visual evidence

Not applicable for a looping CSS animation change — screenshots cannot convey animation continuity. Reproducible steps:

1. Author a workflow JSON with "animation": "trace" in meta
2. deliver and open the HTML
3. Click Live in the bottom toolbar
4. Hover over any node
5. Before: flow plays once (1.15s) and fades out
6. After: flow loops continuously while hovering, stops on Still/unhover

• Automated or browser checks: N/A for animation iteration behavior
• Perceptual visual review: passed (manual, Chrome 137, macOS)

Generated artifacts

Template change affects all future deliver outputs. No existing Gallery, guide, or README artifacts need regeneration — they are delivered artifacts frozen at their build time.

The archify.zip package would need rebuilding if this change is accepted, since assets/template.html is a packaged source.

Checklist

• I used a minimal focused change and preserved existing typed JSON behavior unless the issue requires a contract change.
• I ran the relevant targeted tests and npm test in archify/. — CSS-only change; no test covers animation iteration count
• I added or updated a regression test for behavioral changes. — No existing test seam for CSS animation looping; noted as a gap
• I checked generated artifacts and package freshness when their sources changed. — archify.zip rebuild needed if accepted
• I removed secrets, private repository content, and customer data from fixtures and screenshots.