Regression coverage for preview/export parity

[iocfinc]

Problem or opportunity

The suite introduces new preview and export paths that can regress separately unless parity is covered explicitly.

Proposed change

Add regression coverage for preview/export parity, shared contracts, and toolkit launcher/bootstrap behavior.

Success criteria

• Shared contracts have automated tests
• Preview and export flows are covered where behavior can diverge
• Motion regression coverage remains intact after suite extraction

Constraints or scope notes

• Epic: Epic: Cross-cutting hardening #17
• Focus on high-value parity and contract tests first

[github-actions]

Issue intake checklist:

• Run agent/skills/read_issue.md.
• Classify the issue as Feature, Bug, Documentation, or Refactor.
• Record priority and affected components.
• Keep the lead agent focused on orchestration and choose bounded sub-agents from skills.md for non-trivial work.
• Route feature work to agent/skills/plan_feature.md.
• Route bug work to agent/skills/triage_bug.md.
• For frontend-facing work, plan browser debugging and screenshot validation before PR prep.

[iocfinc]

Status

Partial hardening is ready for review; broader preview/export parity coverage should continue in later slices.

Bug Summary

Preview stability regressions were still possible around startup playback, zoom semantics, shell height ownership, and long control-pane interactions.

Root Cause

The preview behavior depended on implicit defaults across state, render-loop scheduling, shell sizing, and shared accordion behavior, so one fix could leave another movement path open unless the contracts were tested together.

Fix Summary

This branch adds regression coverage for paused startup, zoom-fit reset, viewport-locked shell sizing, and single-active control sections so the preview no longer drifts or resizes during the reproduced Motion Toolkit flows.

Validation

• npm run test:ci
• npm run build
• Browser QA confirmed no preview shift while toggling the long Background controls.
• Screenshot capture was blocked in the sandbox, so final visual proof should use Chrome DevTools MCP locally.

Pull Request

• Shared editor palettes, preview stability, and workflow refinements #41