feat(playground): AI deck-editing tool layer

[leoafarias]

Intent

Stage 1 of 2 for the "edit this deck with AI" session: the tool layer only — the seven-operation contract that lets a model read and mutate the live deck. The conversation UI follows in a dedicated PR.

This is the reimplementation the repo asked for in 492a9bc, which removed the first (unwired) version of these tools and left a spec to rebuild them. The full feature was built on feat/playground-framework-support-v2, but #102 rewrote the playground's AI subsystem underneath its conversation layer (wizard-state-driven view model, deleted chat UI, new style contract). The tool layer is the part that is independent of that churn, already tested, and durable — so it lands first, and the UI is rebuilt on the current subsystem separately (analysis preserved in the workspace; WIP port on wip/deck-editor-on-main).

What's here

• DeckMarkdownCodec — one owner for editor Markdown ↔ canonical slides. MemoryDeckLoader now decodes through it, so the live preview and the deck tools can never parse the same document differently. That invariant is what EditorDeckStore's synchronization barrier relies on.
• DeckStore + EditorDeckStore — canonical document operations bridging the editor document and the rendered preview; each write completes only after the preview observes it (reactive barrier with timeout).
• DeckToolsService — slide operations serialized through one error-recovering FIFO queue; readSlide/updateStyle are injectable seams.
• DeckToolsAdapter — six Dartantic tools (getDeck, createSlide, updateSlide, deleteSlide, moveSlide, readSlide) with Ack-validated arguments and stable wire error codes; slides cross the boundary keyless.
• DeckSlideReader — captures one rendered slide as a PNG thumbnail.

Deliberately not here: the updateStyle tool registration. Its schema and applier targeted the pre-#102 style contract (DeckStyleType, deleted in #102); it returns with the UI PR on the brand/theme contract. The service keeps the seam so the contract is already tested.

Error-translation sites use Error.throwWithStackTrace so typed tool errors keep the original stack trace.

Impacted packages

• playground only.

Verification

Check	Result
dart analyze (playground)	no issues
dcm analyze . --fatal-style --fatal-warnings	no issues
flutter test (playground)	444 passed (411 baseline + 33 new)
dart format	0 changed

[github-actions]

Visit the preview URL for this PR (updated for commit 1f2a8d2):

https://superdeck-dev--pr106-feat-deck-edit-tools-xif2rmds.web.app

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