ipynb attachments: preserve image titles, resolve files/ output URLs

[mmcky]

Fixes #3
Fixes #4

Housekeeping batch for the ipynb attachment-embedding edge cases from PR #1's review trail (plus the async-read recommendation from #10, which is being closed alongside).

Changes

1. Image attachment rewrite drops optional image title #4 — image titles preserved. embedImagesAsAttachments matched the optional "title" in ![alt](url "title") but dropped it on rewrite; the title group is now captured and re-emitted: ![alt](attachment:name "title"). (Note: titles can still be dropped earlier in the pipeline at parse time — that's outside this serializer fix's scope and Image attachment rewrite drops optional image title #4's report.)
2. Image attachment embedding: resolve files/ output folder URLs #3 — files/ output-folder resolution. collectImageData resolved URLs only against the source file's folder, but finalizeMdast rewrites generated images (e.g. executed notebook outputs) to files/... paths that exist only under the export output folder — those were silently skipped. Resolution now falls back to the output folder.
3. Review of PR #1: CommonMark ipynb export + image attachment embedding #10 item 3 — async reads. collectImageData uses fs.promises.readFile with Promise.all instead of synchronous reads (and drops the existsSync TOCTOU check in favor of try/read).

Verification

• New unit test for title preservation (myst-to-ipynb: 58 tests green)
• Manual end-to-end: built two projects with the local CLI — a static image (embedded, as before) and an image existing only under out/files/ (previously attachments: [], now embedded as attachment:dot.png)
• Full monorepo build + test pass

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR tightens MyST→IPYNB image attachment embedding to handle a few edge cases encountered during notebook export, improving fidelity (titles) and correctness (resolving files/ output assets) while also making image reads non-blocking.

Changes:

• Preserve optional Markdown image titles when rewriting image links to attachment: references.
• Resolve attachment image paths against both the source folder and the export output folder (to catch files/... assets generated during export).
• Switch image file reads to async (fs.promises.readFile) with concurrent processing.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/myst-to-ipynb/tests/attachments.spec.ts	Adds a unit test ensuring image titles are preserved during attachment rewrite.
packages/myst-to-ipynb/src/attachments.ts	Captures and re-emits the optional "title" portion in rewritten attachment: image references.
packages/myst-cli/src/build/ipynb/index.ts	Makes image collection async and adds output-folder fallback resolution for files/... URLs.
.changeset/ipynb-attachment-fixes.md	Adds a patch changeset documenting the fixes for both myst-cli and myst-to-ipynb.

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