A diff marks the changed words inside a changed line

[a-team-app]

Context

Part of #179. A changed line in the diff tab is tinted end to end, so you find the actual edit by comparing both sides by eye. This marks the changed words inside a changed line with a stronger background — deeper red on the left, deeper green on the right — like GitHub, JetBrains and delta.

Acceptance criteria

Open a diff (e.g. a file against HEAD) containing edited lines and check:

• On a line that was edited (shown on both sides), the words that differ have a stronger background than the rest of the line: deeper red on the left, deeper green on the right. The rest of the line keeps today's tint.
• Syntax colours on marked words are unchanged; only the background differs.
• A word is a run of letters, digits and _, a run of whitespace, or a single other character: changing count to total in Log(fmt, count) marks only count / total, not the comma or parentheses.
• Re-indenting a line (only leading whitespace changed) marks nothing extra; a whitespace change in the middle of a line is marked.
• A line that was mostly rewritten (the two sides share less than 40% of their characters) gets no extra marking, just today's tint.
• Lines only on one side (added or deleted) look exactly as today.
• Marking stays correct after scrolling sideways, and after the diff refreshes (e.g. after reverting a hunk or editing the file).
• Every bundled theme shows the marked words clearly and readably, including the 16-colour ones (Borland, Turbo Pascal) and the light one (Daylight).
• A user theme that sets VS Code's diffEditor.removedTextBackground / diffEditor.insertedTextBackground uses those colours; one that doesn't still shows marking.

Tests

• The word splitter: identifiers, punctuation, whitespace runs, Unicode letters, empty lines.
• The similarity check and the 40% cutoff, at and either side of the boundary.
• Word ranges produced for typical edits: one word changed, two separate edits on one line, insertion at start/end, leading-whitespace-only change.
• Ranges are computed once per diff/refresh, not per draw.
• Theme colour lookup with and without the two keys.
• AGENTS.md § diff tab updated: what's marked and when.

Out of scope

• Character-level marking, or a setting to switch words / characters / off.
• Scrolling sideways to a marked word on Alt+↓ (decided against).
• Smarter pairing of removed/added lines (today the n-th removed pairs with the n-th added).
• Syntax-aware / structural diffs.
• Marking in the editor or its gutter — diff tab only.
• The page-step change (separate task).

[a-team-app]

Draft PR #373 is up.