feat(video): add dissolve, push, iris, and reveal transitions

[CodeWithDennis]

Summary

• Add selectable video transitions beyond wipe: Dissolve, Push, Iris, and Reveal
• Keep Wipe as the existing layout-aware split-mask animation (vertical / horizontal / diagonal styles still apply)
• Wire the same transition into Play preview, MediaRecorder export, and saved presets

Key changes

• compositor.js: new renderVideoFrame(options, transition, progress)
  • Wipe continues to animate splitPosition through the current mask
  • Other modes cache full A/B frames (labels off), composite by progress, then paint labels once on top
  • A/B frames are cached across frames for smoother preview/export
• video-export.js: recordWipeVideo generalized to recordTransitionVideo (compat alias kept)
• app.js + app.blade.php: Transition control in the Video export panel; presets store videoTransition
• README: document the transition video options

Behavior / impact

Transition	Behavior
Wipe	Layout split animates A→B (including wavy/zigzag diagonals)
Dissolve	Opacity crossfade
Push	Slides along layout axis (horizontal layout → vertical push; otherwise horizontal)
Iris	Circular reveal of B from center
Reveal	Expanding rectangle of B from center

• A → B / B → A still applies to every mode
• Video export remains limited to split layouts (not Overlap)
• Default stays Wipe for existing workflows

Test plan

• Select Export → Video on Vertical / Horizontal / Diagonal
• Confirm Transition shows Wipe, Dissolve, Push, Iris, Reveal
• Play preview for each transition (A→B and B→A)
• Export MP4/WebM for wipe and at least one full-frame mode; verify playback
• Diagonal + Wipe still respects wavy/zigzag/scallop styles
• Labels stay fixed on top during dissolve/push/iris/reveal
• Save preset with a non-wipe transition, reload, confirm it restores
• Overlap still blocks video with the existing helper message
• npm run build succeeds; hard-reload NativePHP/browser to pick up assets