Compare literal Markdown off the main thread

[hweihwang]

Part 4 of the version-comparison work for nextcloud/collectives#599: a literal Markdown source comparison, for what a rendered view cannot show.

• Source view with line numbers and token-level emphasis; diagnostics for line-ending changes, per-line LF/CRLF badges, trailing whitespace, missing final newlines
• All computation in a cancellable module worker with three explicit limits (2,000,000 characters, 50,000 edits, 2,000 ms). A limit always reports itself as limited, never as "identical"

Depends on: #9049 (base review/structured-markdown-comparison-documents). Last Text unit; then nextcloud/collectives#2699 and #2700.

Full story, screenshots and the test matrix live in the #599 implementation update: nextcloud/collectives#599 (comment)

AI disclosure: developed with OpenAI Codex (gpt-5.6-sol). Codex generated the implementation and tests under my direction; I reviewed, cleaned and verified the code, ran the full test and CI matrix, and signed the commits personally.

[benjaminfrueh]

/backport to stable35

[backportbot]

The backport to stable35 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable35
git pull origin stable35

# Create the new backport branch
git checkout -b backport/9050/stable35

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick b5ff1b0c

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/9050/stable35

Error: Failed to check for changes with origin/stable35: No changes found in backport branch

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.