feat: add playground app and supporting framework changes

[tilucasoli]

Summary

• Introduces packages/playground — a standalone Markdown editor + presentation app (forerunner of SuperDeck Studio) that runs without the CLI pipeline.
• Adds the framework hooks needed to host SuperDeck slides inside a custom app (nullable slide chrome, MixScope inheritance, thumbnail invalidation).
• Fixes a Hero shuttle bug where text resized at handoff because DefaultTextStyle was not inherited into the Navigator overlay.

Changes

packages/playground/ (new)

Standalone editor with rich-text Markdown input, live thumbnail previews, full-screen presentation mode, and system theme support. Depends on local superdeck, superdeck_core, and superdeck_builder. Not published (publish_to: none).

packages/superdeck

• SlideParts.header / footer now nullable so embedded hosts can omit chrome.
• SlideRenderView uses MixScope.inherit, and SlideCaptureService re-applies SDColors.colorMap so captured frames resolve the same tokens as live slides.
• DeckPresentationState.deleteAllThumbnails() + ThumbnailService.deleteAllThumbnails() for editor flows that need to invalidate the full cache.
• Hero shuttle re-applies the source slide's DefaultTextStyle so text properties left unset by element specs no longer resolve against the bare overlay default.

packages/builder

• Re-export comment_parser, markdown_parser, section_parser so downstream apps (playground) can drive the build pipeline directly.

demo/ and packages/superdeck

• Bump google_fonts from ^6.3.2 to ^8.1.0 (matches playground).

Test plan

• melos run analyze
• melos run test (new tests for deleteAllThumbnails, ThumbnailService.deleteAllThumbnails, and Hero shuttle text-style behavior)
• Manually run playground: cd packages/playground && fvm flutter run -d macos
• Manually run playground on web: fvm flutter run -d chrome
• Verify demo app still builds and the new google_fonts version works
• Verify Hero transitions in the demo don't resize text mid-flight

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