feat: add shared element transitions

[milkevich]

Summary

Adds shared element transitions to Routeveil as a new page-transition capability.

Shared elements are matched across routes by name and run in three sequential phases:

Basic logic:
page exit → shared element movement → page enter

This PR includes:

• RouteveilSharedElement public API and types
• Shared element registration, matching, cloning, animation, handoff, cleanup, and interruption handling
• Support for multiple shared elements and different source/target element types
• Safe fallbacks for missing or ambiguous targets
• Reduced-motion, SSR, accessibility, and lifecycle coverage
• Shared element documentation and README updates
• A link to the shared elements playground
• New /lab/shared-elements masonry gallery demo
• SEO, sitemap, and machine-readable documentation updates
• Package version update to 0.2.0

Existing page transitions, overlay transitions, and same-page playback remain unchanged when shared elements are not used.

Validation

• npm run check
• npm run build:demo
• Tests cover new or changed behavior
• Public documentation matches verified behavior
• Public API and package contents were reviewed
• Visual styling and layout are unchanged unless required by the change

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Project	Deployment	Actions	Updated (UTC)
routeveil	Ready	Preview, Comment	Jul 25, 2026 4:44am