feat(preview): add resolveFamily to keep custom font families in svg mode

[baseballyama]

Summary

renderSlideToSvg(…, { textLayout: 'svg' }) wrote every font other than the Calibri / Cambria / Arial / Times / Courier families as Carlito, so a host that shows the SVG as a sanitized <img> and embeds its own @font-face rules could not get Lora / Inter / etc. applied. This adds RenderSlideOptions.resolveFamily, which maps each authored font name to the family written to <text font-family> and passed to measureText.

Motivation

Closes #461

The hook already existed internally (SvgTextArgs.resolveFamily, used by the audit path and the foreignObject path); this exposes it for svg mode, as proposed in the issue. A fonts: [{ family, src }] option that also embeds @font-face was the alternative, but it would put font loading inside the library; the host already owns that.

Changes

• New optional RenderSlideOptions.resolveFamily?: (family: string | null) => string. Consulted only when textLayout is 'svg'; defaults to substituteFamily, so output is unchanged when it is not passed.
• It applies to shape text, bullets, table cell text, and the shrink-to-fit (<a:normAutofit/>) search, so the measurer sees the same family for fitting and for laying out.
• A returned name that is not a sequence of CSS identifiers (e.g. M PLUS 1p) is quoted in the SVG; otherwise the browser would drop the whole font-family value. The measurer still receives the bare name. The default substitute names are identifiers, so default output is unchanged.
• The foreignObject path ignores it — it already writes the authored name into a CSS font stack.
• Fix in auditTextLayout: its shrink-to-fit search measured with the substitute family while its layout pass used the authored one; both now use the authored family.
• Out of scope: the Node raster entry (renderSlideToImage) doesn't take the option yet — resvg matches a font file's internal family name, so keeping the emitted name, the resvg name and the measurer key consistent needs its own design.
• README: documents svg mode and the new option.

Testing

• test/preview-resolve-family.test.ts: default substitution unchanged; kept family is written to the SVG and handed to the measurer; table cells; autofit search measures with the resolved family; foreignObject output is identical with or without the option. quoting of non-identifier names; audit fit/layout family consistency. Each fails without the corresponding change (the autofit one fails if only the fit threading is removed).
• npx vitest run (full suite), oxlint, oxfmt --check, tsc --noEmit pass locally.

Breaking changes

None

Checklist

• I have read CLAUDE.md and followed the project's conventions.
• I have added or updated tests for the change.
• I have added or updated documentation where user-visible behavior changed.
• If this is a breaking change, I have added a changeset / CHANGELOG entry and
  flagged it above.
• I have re-read my own diff and removed dead code, debug prints, and stale
  comments.
• If I used an LLM to draft this PR, I have verified each change myself, this
  PR represents real work that warrants a maintainer's review, and I am
  willing to defend each line in review.

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