Embed interactive playgrounds in the web thread

[time-attack]

Refactors interactive web playgrounds into typed, scope-authorized file artifacts while preserving the original contribution and authorship.

• removes prose directives and response-time document rewriting
• limits v1 to embedded play, open, and raw source
• isolates playgrounds with browser sandboxing and a restrictive content policy
• verification: typecheck, lint, focused tests, Web UI suite, production build

[time-attack]

Live web-thread take: new chat, asked to explain integrals and drag the rectangles. The playground card paints in the thread; then the slider, left/mid/right sampling, and the chart are driven live.

explain-integrals.mp4

[time-attack]

@16francej

[16francej]

Superseded by #701, which rebased this work onto current main, preserved the original feature commit and authorship, addressed the CI/security findings, and has now merged. Thanks @time-attack for the contribution.