fix(preview): render EMF artwork and order gradient stops

[baseballyama]

Summary

Restore EMF vector artwork in imported slide previews and render gradient stops in position order. A closing slide with a skyline now shows its transparent white artwork over a smooth blue background instead of an image placeholder and a horizontal color seam.

Motivation

A user reported the discrepancy between PowerPoint and the preview and supplied a reproduction file directly, so there is no separate issue. The file uses an EMF image and gradient stops serialized in the order 52%, 0%, 100%. The private presentation is not included; regression fixtures are generated in the tests.

Changes

• Render supported EMF solid-filled line and Bézier paths as transparent SVG, including window/viewport transforms and clipping. Unsupported drawing records, EMF+ content, and malformed input retain the image fallback.
• Sort linear and radial gradient stops without mutating the source presentation.
• Document the supported EMF subset and add a patch changeset for the preview package. No dependencies or public exports are added.

Testing

• pnpm exec vitest run test/preview-imported-artwork.test.ts test/preview-effects-fills.test.ts test/preview-node-raster.test.ts: 33 tests passed.
• Rendered the supplied presentation before and after the change and visually checked the final slide's skyline and background. The reproduction file and rendered images remain local.
• New tests cover transparent SVG/Node raster output, unordered linear/radial stops, signed 16-bit paths, clipping/transforms, buffer offsets, malformed records, and unsupported content.
• Full static checks, build, and the remaining test suite run in CI.

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.