Scope can-play composition with built-ins

[spencerc99]

Summary

• Scope bare can-play DOM props away from built-in capabilities when an element uses both.
• Add regression coverage for can-play + can-move composition.
• Document the supported composition behavior and add a patch changeset.
• Fix TypeScript nullability around SyncedStore/page-data proxy access found during package build.
• Leave starter templates unchanged because their current examples do not demonstrate multi-tag composition and remain copy-pasteable.

Root cause

The runtime read generic DOM element props for every capability tag on a node, so can-play props like defaultData and updateElement could override a built-in initializer such as can-move.

Verification

• ./node_modules/.bin/vitest run --no-typecheck in packages/playhtml (31 files, 284 tests passed; existing perf stdout and expected can-duplicate stderr)
• bun run build in packages/playhtml (passed with existing Sass legacy API warning)
• /Users/spencerchang/Projects/playhtml/node_modules/.bin/astro build in apps/docs (passed with existing Vite/Astro warnings)
• git diff --check

[pkg-pr-new]

Open in StackBlitz

bun add https://pkg.pr.new/@playhtml/common@246

bun add https://pkg.pr.new/playhtml@246

bun add https://pkg.pr.new/@playhtml/react@246

commit: 2b9149a

[cloudflare-workers-and-pages]

Deploying playhtml with    Cloudflare Pages

Latest commit:	2b9149a
Status:	✅  Deploy successful!
Preview URL:	https://08660d27.playhtml.pages.dev
Branch Preview URL:	https://cx-can-play-capability-compo.playhtml.pages.dev

View logs

[spencerc99]

Code review

No issues found. Checked for bugs and project-instruction compliance.