fix(pptx): preserve 8-digit hex (#RRGGBBAA) alpha in gradients and solid fills

[karthikmudunuri]

What & why

Translucent fills written as 8-digit hex (#RRGGBBAA) were serialized opaque. The cause was in the serializer's parser, not its emitter:

• parseFill normalized #-prefixed colors through hexBare, which truncates to 6 digits — so the alpha byte of #RRGGBBAA (and #RGBA) was discarded before it could reach <a:alpha>.
• The emission side (gsLstXml, solidFillXml) already handled GradStop.alpha correctly, so only alpha arriving via rgba(...) survived. That's why a deck full of #RRGGBBAA gradient stops produced essentially one <a:alpha> in the whole file — a transparent fade rendered flat/opaque.

Changes

• pptxWriters.ts — new parseHexColor helper (replaces normaliseHex) extracts alpha from 4-digit (#RGBA) and 8-digit (#RRGGBBAA) hex. parseFill returns it, so alpha now flows to gradient stops (via parseStops), solid fills, and slide backgrounds.
• deckToPptx.ts — addShape maps a solid fill's alpha to pptxgenjs transparency (0 opaque … 100 transparent) so flat translucent shape fills no longer render opaque. (Gradient/image fills already took the synth path.)
• synth-writers.test.ts — 3 new tests: per-stop alpha from #RRGGBBAA linear stops, solid-fill alpha, and radial-gradient <a:gradFill><a:path path="circle"> emission.

Scope notes

• Radial gradients were already emitted correctly — added a test to lock it in.
• Font embedding is not changed: synthesiseEmbeddedFonts + <p:embeddedFontLst> already fire from deck.fonts. The "fonts fall back to Calibri" symptom for template-built decks is a font-sourcing issue (the template JSON carries family names but no binaries, so deck.fonts is empty) and can't be fixed inside the serializer — it needs the template pipeline to attach font binaries, or the brand fonts installed in the render container.
• Numeric-weight → named-face mapping (300/500/600 → Light/Medium/SemiBold) is deferred; decks here use 400/700, which map cleanly to the bold flag.

Release

Includes a changeset (@textcortex/slidewise: patch) so the release workflow versions and publishes it.

Testing

pnpm test in packages/slidewise: 51 passed, 3 skipped (pre-existing). Typecheck (tsc -p tsconfig.lib.json) clean. (Lint via eslint . fails to resolve @eslint/js from the repo root — a pre-existing workspace setup issue unrelated to this change.)