Docx Image Cleaner Coverage Pass

Goal

Add a narrow, high-value non-React coverage pass for @platejs/docx focused on the image and RTF cleaner helpers:

• cleanDocxImageElements.ts
• getRtfImageHex.ts
• getRtfImagesMap.ts
• getRtfImagesByType.ts
• getRtfImageMimeType.ts

Constraints

• Fast-lane only.
• No /react.
• No broad package sweep.
• Prefer pure helper specs, plus one honest DOM contract spec for image replacement.

Slice

1. Add pure helper specs for mime, hex, grouped image extraction, and map precedence.
2. Add a DOM cleaner spec for:
   • external URL fallback via alt
   • local file replacement from RTF image data
   • unknown image removal
   • VML image replacement path
3. Run targeted tests, profile, fast-budget gate, then package build/typecheck/lint.

Notes

• Existing docx coverage is still thin around RTF image parsing.
• getVShapes.spec.ts currently contains a fake placeholder assertion, but this slice does not need to widen into that file unless the new tests expose a real dependency gap.
• During execution, the new map tests exposed a real bug: getRtfImagesByType(..., '\\shp') was also matching \\shppict blocks because the helper split on raw substrings instead of exact RTF control words.