Add Diff Checker tool

[02karthik]

What is this tool?

Diff Checker compares two versions of any text and shows exactly what changed — additions and deletions highlighted inline. It offers two granularities: Words (best for prose — inline swift replaces quick) and Lines (git-style, best for lists/code), with a live added / removed count. It's aimed at Hillnote's writing/research audience, who constantly compare drafts, edits, and revisions.

The diff is a pure-JavaScript LCS (longest-common-subsequence) edit script — no library, no network — and a guard falls back to line mode on very large inputs so typing stays snappy.

Checklist

• It's a single file: diff-checker/index.html with all CSS + JS inline.
• It is fully self-contained / offline — no remote fonts, scripts, styles, images, fetch, or import.
• No analytics, trackers, or phone-home behavior.
• Added a screenshot at screenshots/diff-checker.webp (via npm run screenshot diff-checker).
• Registered the tool in tools.json (dir matches the folder name).
• Bumped the top-level catalogVersion (23 → 24).
• For an update to an existing tool: N/A — new tool at version: 1.
• npm run validate passes locally.

Notes for reviewers

• Design system: single azure accent for chrome; the green/red on the diff is the documented semantic palette (positive/negative families), used only for illustrative diff content. Every colour goes through a token — verified in both light and dark (prefers-color-scheme).
• Not colour-only: additions are underlined (<ins>) and deletions struck through (<del>), so the diff is legible without relying on colour.
• First-run example: seeds a short worked example so the tool (and gallery card) shows what it does; cleared state is stored as "" so the sample never reappears once you edit the panes. State is namespaced under diff-checker:.
• Performance guard: MAX_TOKENS caps the word-level DP; beyond it the tool auto-switches to a line diff with a notice, and a hard ceiling shows a "too large" message rather than freezing.
• Distinct from existing/closed work: no existing tool does text comparison, and this is unrelated to the previously closed Markdown Preview / Markdown Table PRs.

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