Compare literal Markdown off the main thread

[hweihwang]

In one sentence: the Markdown source view that catches what rendering hides — line endings, whitespace, syntax — computed in a worker.

What and why

Two revisions can render identically and still differ: LF vs CRLF, trailing whitespace, a missing final newline, link syntax rewritten. The semantic engine works on parsed documents, so by design it cannot see these. This PR adds the view that can.

What's in this PR

• A Markdown source view comparing the literal text, with line numbers and token-level emphasis
• Explicit diagnostics: 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"

Zero rendered changes, stated honestly — the whole reason this view exists.

The source view. Source change groups can differ from rendered ones.

Explicit diagnostics for what the rendered view cannot show.

Design notes

• Source is a separate subsystem, not a mode — the semantic engine can't see what parsing discards, and that is the entire subject of this view.
• The worker exists even for small documents: Myers complexity tracks edit distance, not size, so a size cap alone would not stop a two-second UI freeze.
• The worker is emitted under js/ (verified: js/markdownSourceComparison.worker-Bn7Bh2ec.worker.mjs, no assets/) so the post-merge asset pipeline regenerates it. Under Vite's default assets/ it would build locally but disappear after merge.
• diff@8.0.3 (BSD-3-Clause, kpdecker/jsdiff) is declared here only — T1–T3 don't import it.

Stack

Depends on: #9045. Last Text unit — after this, Collectives nextcloud/collectives#2697 and #2698. Opened stacked — the diff narrows as predecessors merge; I'll rebase before review.

Verification

ts:check and lint pass; complete suite 1,917/1,917. Worker ready/limited protocol responses are tested against the real module. Production build re-run at this tip: emitted js/markdownSourceComparison.worker-Bn7Bh2ec.worker.mjs, no assets/ directory.

AI assistance

Commits carry Assisted-by: Codex:gpt-5.6-sol; an independent review pass was run with Claude Opus 5. Description drafted with Hermes (deepseek-v4-pro) from my notes, then reviewed by me.

Related

• Show differences (added/removed/changed text) when checking past versions collectives#599
• Version difference extraction #4422