feat(pptx/render): applyEdits layout-instantiation + headless render API + font transparency

[karthikmudunuri]

Stacked on #95 (applyEdits). Base = karthikmudunuri/work-on-plan — review/merge #95 first, then this rebases onto main.

This PR now covers all three remaining release-bundle items: P0 layout-instantiation (incl. non-text placeholders), P1 headless render API, and P2 font transparency.

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P0 — layout-instantiation inside applyEdits

Closes the one real lossless gap: applyEdits previously deferred source: { layoutId } to the lossy serializeDeck path. Now a PlannedSlide can clone a source slide ({ slideIndex }) or instantiate a fresh one from the template's own layouts ({ layoutId, fills? }). Because the layout is already a part of source, instantiation is still a lossless patch — the new slide binds to ppt/slideLayouts/<layoutId>.xml (inheriting theme/master/background chrome) while every other part stays byte-identical. That's lossless and scale-with-variety in one deck.

Every placeholder (text and non-text) is materialised as an addressable element with the deterministic id layoutSlotElementId(layoutId, key) (exported), key = the placeholderKey / summarizeLayouts slot key:

• text / obj → <p:sp> + <p:txBody>, populated from fills, editable via setText
• picture → <p:pic> with a transparent placeholder blip so setImage can repoint it
• chart / table / other → positioned <p:sp> exposing slot geometry (host fills via addChart / addDiagram)

Geometry is read EMU-native from the layout (falling back to the matching master slot). Unresolvable layoutId → onWarning + skip.

P1 ⭐ — renderDeckToImages (headless visual-QA render)

Browser-free renderer (no Playwright/Chromium/DOM) that draws what the editor draws — native charts via buildChartOption + ECharts SSR, diagrams via layoutDiagram, text/shapes/images/backgrounds in z-order — not the OOXML raster fallbacks.

• renderDeckToSvg(deck, opts?) → one composed SVG per slide (ECharts loaded on demand, so it never bloats the editor bundle — main chunk stays ~1.1 MB).
• renderDeckToImages / renderSlideToImage / renderPptxToImages → raster bytes.
• Rasterisation is an injected hook (opts.rasterizeSvg, e.g. @resvg/resvg-js); the default tries a dynamic resvg import and throws a clear error if it isn't installed — no hard native dep.
• opts: slides (1-based subset), dpi (canvas scales by dpi/96), format, maxWidth. Deterministic (no animation).

Enables: applyEdits → renderPptxToImages → model inspects → targeted fix → re-render.

P2 — font transparency (deck.fontUsage)

parsePptx now stamps deck.fontUsage: { family, embedded }[] — every font family the text uses, flagged whether the source embeds it (<p:embeddedFontLst> → a real ppt/fonts/* part) or merely references it. Hosts warn at generation time on missing brand fonts. Read-only diagnostic, distinct from deck.fonts (the embeddable payloads).

Note: kept as a new field rather than re-typing deck.fonts, which is load-bearing (the serializer reads its byte payloads to re-embed).

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Tests (all new)

• apply-edits-layout.test.ts — mixed clone + layout-instantiate (3 slides, correct layout rel, chrome inherited, cloned parts byte-identical, zero dangling rels), master-geometry inheritance, unresolved-layout skip.
• render-deck.test.ts — chart slide carries the real series colour, diagram nodes + image present, 1-based subset, injected rasteriser order, maxWidth cap, no headless-browser import in the source.
• font-usage.test.ts — embeds font A + references-only font B → fontUsage reports {A:true}, {B:false}.

Verification

• pnpm typecheck ✅
• pnpm test ✅ (179 passed, 9 skipped — fixture-gated)
• pnpm --filter @textcortex/slidewise build:lib ✅ (no warnings; echarts split into its own lazy chunk)

Three changesets + README sections included.

Out of scope (remaining from the earlier bundle, not requested here)

P0b addSlideFromLayout non-text instantiation on the serializeDeck path; F1/F2 image fixes (the plan notes those land in #94).