docs: plan — PPTX round 2 (table styles, charts, EMF fallbacks)

[karthikmudunuri]

Planning PR — no code changes. This is a scoping document for the next round of PPTX importer work, opened so the architecture and ordering can be reviewed before implementation starts.

What's in this PR

A single file at `docs/plans/pptx-tables-charts-emf.md` proposing three deferred-from-#36 items as the next round:

1. Table styles — parse `ppt/tableStyles.xml` and resolve `<a:tblPr><a:tableStyleId>` so styled tables stop rendering with default flat fills. Smallest item, ships first.
2. Charts — render the cached PNG that PowerPoint embeds alongside every `<c:chart>` so bar / pie / line / column charts appear (visually accurate, not live-editable). Live chart rendering becomes a follow-up PR if editability inside Slidewise is needed.
3. EMF / WMF fallback — when a `<p:pic>` references EMF/WMF and a raster sibling is shipped in the deck, render the raster. True EMF→SVG decoding stays deferred (needs a WASM port; multi-day project).

What this isn't

• Live chart rendering (parse series, draw via ECharts) — flagged as a separate follow-up.
• A true EMF/WMF decoder — flagged as a separate WASM project.
• Embedded TTCOMPRESSED fonts, animations, SmartArt — all explicitly out of scope.

Open questions raised in the doc

1. Are we OK with cached-image charts (faithful but read-only) as the first step, or should we land live rendering directly?
2. Should the EMF fallback path expose a host hook (`onMissingMedia(rId)`) so apps that process decks server-side can supply pre-rasterised replacements?
3. Do we need to honour `<a:tableStyleList def="…">` when a table has no `tableStyleId`?

How to proceed from here

• Comment on the plan with thoughts on the open questions.
• Once aligned, close this PR (it's docs-only and the implementation PR will subsume it) or merge it as a permanent planning artefact under `docs/plans/`.
• Implementation work happens on a new branch off main targeting one PR per the three commits, releasing as 1.10.0.

Test plan

Doc-only change, no tests to run.