feat: render loading placeholder for incomplete streaming images

[sleitor]

Summary

Fixes #503 — incomplete images during streaming now render an animated skeleton placeholder instead of being silently removed. This mirrors the existing behavior for incomplete links.

What changed

---

• link-image-handler.ts: Both handleIncompleteUrl() and handleIncompleteText() now emit ![alt](streamdown:incomplete-image) for incomplete images instead of stripping them entirely
• index.ts: Updated earlyReturn check to also short-circuit on streamdown:incomplete-image; fixed JSDoc for images option
• Tests updated: removal expectations → placeholder expectations

packages/streamdown

• lib/image.tsx: ImageComponent detects src === 'streamdown:incomplete-image' and renders an animate-pulse skeleton div with data-incomplete="true"
• Tests added for the new placeholder behavior

Changeset: remend@patch + streamdown@patch

Before / After

Before: ![logo](./assets/log → (image silently removed, layout shift)

After: ![logo](./assets/log → animated grey skeleton placeholder shown until the stream completes

Test coverage

• 356 remend tests ✅
• 964 streamdown tests ✅

All existing tests updated; 4 new tests added for the placeholder behavior.

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[farnabaz]

Thanks for the PR @sleitor
Do you mind removing line numbers commit from this PR?

Also I do think streamdown schema will be blocked by rehype-harden package we need to change placeholder to another text. maybe even an 1x1 pixel placeholder

data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" width="1" height="1"/>
// or equivalent png

[sleitor]

Thanks for the review, @farnabaz!

• Removed the unrelated lineNumbers commit that got mixed into this branch (rebased, now only the placeholder feature + this fix remain).
• Replaced the streamdown:incomplete-image custom-scheme marker with a 1x1 transparent PNG data: URI (INCOMPLETE_IMAGE_PLACEHOLDER, exported from remend). This should survive strict URL-sanitizing schemas like rehype-harden, since only http(s)/data schemes are typically allowlisted — a bare custom scheme string would likely get stripped before it reaches the ImageComponent's src check.

All 356 remend + 951 streamdown tests pass, lint is clean. Let me know if you'd like a different data URI (e.g. transparent GIF/SVG) or any other adjustments.