fix(pptx): keep source bytes resolvable after state cloning

[karthikmudunuri]

Summary

1.12.0 shipped the master / layout / theme / font / EMF / slide-bg preservation pipeline — but it was silently no-op'ing in practice. The pipeline depended on a non-enumerable __slidewiseSourcePptx attachment on the deck. structuredClone (used by snap() for history) AND every { ...deck, ... } reducer spread strip non-enumerable properties. So:

1. User imports PPTX → parsePptx attaches source bytes (non-enumerable)
2. User edits one text → reducer spreads { ...deck, slides } → source attachment gone
3. Editor saves → resolveSource returns undefined → preserveUnknowns bails immediately → pptxgenjs writes a stripped-down deck

End user sees: 28 layouts → 1, 5 embedded fonts → 0, half the slides empty. Exactly what they reported on 1.12.0.

Fix

Move the source attachment to an enumerable identifier on the deck, with bytes held in a module-level cache:

• parsePptx stamps deck.sourcePptxId = nanoid() (enumerable, JSON-safe string).
• A module-level Map<string, ArrayBuffer> caches source bytes keyed by that id.
• resolveSource consults the cache when no explicit options.source is passed, falling back to the legacy non-enumerable attachment for direct callers.

structuredClone, spread, JSON.parse(JSON.stringify(deck)) all preserve sourcePptxId because it's a plain enumerable string, so the cache lookup keeps working across every reducer mutation.

The cache is in-memory only — cross-session round-trips (page reload → rehydrate from localStorage) still need the host to re-attach source bytes via serializeDeck(deck, { source }). Documented on the field's TSDoc.

Validation

New regression test in chrome-preservation.test.ts: parses eon-deck.pptx, runs the deck through structuredClone + an object spread (mirroring the store's snap() and a reducer), saves with NO explicit source, asserts 28/28 layouts and 5/5 embedded fonts survive.

End-to-end against the user's exact scenario (/tmp/edit_sim_no_source_passed.mjs):

sourcePptxId on deck: waYqwtgvbgv-
Non-enumerable SOURCE_PPTX still there? false
Enumerable sourcePptxId still there? true
Saved zip: { masters: 1, layouts: 28, themes: 3, fonts: 5, slides: 25, media: 18 }
Slides with content: 25/25

Versus the user's broken save on 1.12.0 with the same input: 1 layout / 0 fonts / many empty slides.

Test plan

• pnpm typecheck clean
• pnpm --filter @textcortex/slidewise test — 38/38 passing
• End-to-end save against eon-deck-v1.pptx with no source option produces fully-preserved output

Changeset

Patch bump on @textcortex/slidewise (1.12.0 → 1.12.1) — bug fix on a regression that landed in 1.12.0; no API breaks. Deck.sourcePptxId is a new optional field, additive.