feat(slidewise): add jsonDeck prop for AI-generated decks

[karthikmudunuri]

Summary

• New top-level jsonDeck?: Deck | string prop on SlidewiseEditor / Slidewise.Root. AI emits either a parsed Deck or a JSON string and Slidewise handles JSON.parse + migrate() internally — no host glue needed. Takes precedence over deck; deck is now optional and one of the two must be provided.
• New export resolveJsonDeck(input) — the same parse + migrate helper, surfaced so hosts can validate model output before mounting (or use it in non-React tooling).
• The schema LLMs target is the existing public Deck type — no new shape to learn.

import { SlidewiseEditor, resolveJsonDeck } from "@textcortex/slidewise";

// Pass JSON straight from the model:
<SlidewiseEditor jsonDeck={aiGeneratedJsonString} />

// Or validate first:
const deck = resolveJsonDeck(aiGeneratedJsonString);
<SlidewiseEditor deck={deck} />

SlidewiseFileEditor is intentionally untouched — it sources its deck from a PPTX blob, so the JSON path doesn't apply.

Test plan

• tsc -p tsconfig.lib.json --noEmit clean
• Full vitest suite: 39 passed / 3 skipped (unchanged baseline + 4 new tests for resolveJsonDeck: object input, string input, malformed JSON, future-version rejection)
• Smoke: mount <SlidewiseEditor jsonDeck={...} /> in the website demo with both a Deck object and a JSON string
• Smoke: confirm existing <SlidewiseEditor deck={...} /> callers are unaffected