* test(studio): add design-panel QA fixture and triage matrix
Fixture project covering all panel-editable element archetypes,
plus the QA findings matrix from the design-panel bug campaign.
* fix(studio): make canvas selection hit intended elements
- honor author pointer-events:none in hit-testing (was selecting invisible overlays)
- pause playback before mousedown sampling; fall back to hover selection on null resolve
- invalidate committed selection when the active composition changes
- double-click keeps selection and defers to multi-candidate click cycling
* fix(studio): close remaining selection-layer review findings
- hoverSelection fallback now wired at all 3 mousedown call sites (box-click,
blocked-drag, plain overlay click) instead of just the overlay path
- pointer-events override detection reads computed style, not inline style,
so a CSS-class opt-in (not just inline style=) on a descendant is honored
- defensively remove the pointer-events override before the group-fallback
check too, closing a theoretical gap in the no-elementsFromPoint branch
- a click that resolves to nothing (dead-zone / deselect) no longer leaves
playback paused if it was already playing
