v1.6: diff-on-reload (track-changes review surface)

[nonatofabio]

Summary

v1.6 lands the headline UX leap from the v2 roadmap: when an external edit hits the active file (vim, an agent, anything), Mindle renders the change as a Word-style track-changes overlay you can accept or reject — per chunk, or whole-document.

This is the foundation the v2.0 MCP loop sits on. Today it's already useful on its own — anyone editing the file in vim, Cursor, or Claude Code outside Mindle gets visible change tracking.

What changed

Snapshot model (Swift) — DocumentStore now carries a per-tab lastSyncedText alongside rawText. When the file watcher fires after an external edit, rawText updates but lastSyncedText holds the user's last reviewed version, so the JS layer can compute and render the diff. The sidecar persists lastSyncedText only when there's an in-flight review (so v1.5 sidecars round-trip clean), letting an unfinished review survive close and reopen.

Diff render (JS + CSS) — mindleLoad takes an optional third argument: the baseline. When it differs from the current text, the pipeline:

1. Walks Diff.diffLines(baseline, current) (jsdiff vendored at Resources/web/vendor/diff.min.js) and groups consecutive +/- segments into chunks with start/end positions in both texts.
2. Builds a markdown source that interleaves unchanged regions of the new text with raw-HTML wrappers around each chunk.
3. Pre-renders each chunk's before/after through markdown-it (so lists, headings, code fences inside a chunk look right) and surfaces them as styled blocks with ✓ Keep / ✗ Revert chips.

The print pipeline strips the chunk chrome, so PDF export reflects accepted text only.

Round-trip (Swift ↔ JS) — clicking ✓ Keep posts a new baseline back to Swift via diffSetLastSynced; clicking ✗ Revert posts a new current text via diffSetCurrent, which Swift writes through to disk. Each click triggers a re-render of the now-shrunk diff. No partial state in JS — Swift owns all text.

Whole-doc shortcuts — Edit menu gains Keep All Changes (⌘⌥⏎) and Revert All Changes (⌘⌥⌫). Disabled when there's no in-flight diff. Useful when the agent's revision is bulk-correct or bulk-wrong.

Test plan

• Open a .md file, edit it externally (echo "..." >> file.md or save in vim) — diff chunks render with green/red wash and chips.
• Click ✓ Keep on a single chunk — that chunk's content joins the baseline; remaining chunks still rendered.
• Click ✗ Revert on a single chunk — file on disk reverts that section to the baseline; the diff shrinks correspondingly.
• Click ⌘⌥⏎ (Keep All) — baseline catches up to current; document renders normally without chunk chrome.
• Click ⌘⌥⌫ (Revert All) — file on disk reverts to baseline.
• Close the window mid-review, reopen — sidecar restores the in-flight diff (chunks still show).
• Tab switching between two files mid-review — each tab keeps its own diff state.
• PDF export with diffs in flight — PDF contains only accepted (current) text, no chrome.

Out of scope (v1.6.x or later)

• Word-level highlights inside chunks (currently paragraph-block granularity only).
• Diff statistics / chunk count badge in the UI.
• Keyboard shortcuts to navigate between chunks.
• Inline annotations on chunks (reusing the highlight system to mark chunks for the agent).

Part of v2.0 roadmap.