feat(plan-diff): word-level inline diff rendering

[backnotprop]

Summary

Closes #560.

Replaces the current "whole old block struck-through above the whole new block" rendering with in-context word-level highlights for small edits, while preserving the existing block-level behavior for structural changes. Users can toggle between three modes: Rendered (new default, word-level), Classic (legacy block-level stacked), and Raw (git-style).

Before: a one-word reword showed as the full paragraph twice.
After: just the changed words are struck/highlighted inline; surrounding prose is untouched.

What's in this PR

Engine (packages/ui/utils/planDiffEngine.ts)

• Second-pass word diff on modified blocks that pass a whitelist gate (paragraph / heading / list-item with matching structural fields).
• Sentinel substitution atomizes three patterns before the word diff runs so diff markers can't fragment them:
  • Inline code spans (`foo`)
  • Markdown links ([text](url))
  • Fenced code blocks (variable-length, via backreference)
• Render wrapper picks up the NEW block's orderedStart so renumbered list items display the current plan's numeral.
• Annotation-context helper returns both old and new content for inline-diffed modified blocks so comments on struck-through words preserve that text in the exported feedback.

Renderer (packages/ui/components/plan-diff/PlanCleanDiffView.tsx)

• New InlineModifiedBlock component renders a modified block as a single structural wrapper with <ins>/<del> tags inside. The unified string is parsed through InlineMarkdown once, so markdown delimiter pairs (bold, italic, links) survive across token boundaries.
• InlineMarkdown extended to recognize <ins>/<del> passthrough, recursively parse link anchor text, and include < in the plain-text stop-char scanner.
• Click-to-annotate works in every editor mode, not just comment.

Mode switcher (packages/ui/components/plan-diff/PlanDiffModeSwitcher.tsx, PlanDiffViewer.tsx)

• Third "Classic" tab between Rendered and Raw. Default is Rendered (word-level, marked "exp"); Classic forces block-level stacked fallback for every modified block; Raw is unchanged.

Styling (packages/ui/theme.css, packages/editor/index.css)

• plan-diff-word-added / plan-diff-word-removed utility classes for inline highlights; box-decoration-break: clone so wrapped lines don't smear.
• Inline <code> inside the diff wrappers picks up a tinted background so code-pill changes read unambiguously green/red.
• New plan-diff-modified class (amber border) for inline-diff modified blocks — follows the GitHub / VSCode convention of green=add, red=remove, yellow=both.

Tests (packages/ui/utils/planDiffEngine.test.ts)

• 18 tests covering the qualification gate, structural-field matching, sentinel round-trip (inline code / links / fences), and token content for common edit patterns.

Demo infrastructure

• Default demo in demoPlan.ts gets a "Context" section at the top that showcases the engine (bold phrase swap, inline-code pill swaps, link URL change, single-line code edit). Mermaid + graphviz diagrams moved to an Appendix so they don't clutter the top of the plan.
• dev-mock-api.ts splits its V1/V2 constants into *_DEFAULT (original Real-time Collaboration content, byte-identical to pre-branch state) and *_DIFF_TEST (opt-in 20-case engine stress test). Selected at runtime by the VITE_DIFF_DEMO env var.
• App.tsx reads the same env var so V2/V3 always stay paired.
• demoPlanDiffDemo.ts (new) holds the 20-case stress test as a kept-in-tree dev fixture, documented as opt-in.

Known limitations (documented in demo cases ⑯–⑳)

• Word swap inside a multi-word bold phrase splits the ** delimiter pair — upstream tokenizer issue, requires AST-level diff injection to fix.
• Literal <ins> / <del> HTML tag text in plan prose can't be disambiguated from injected diff markers — rare; documented.
• Renumbered ordered list items (same text, different number): visually rendered as a modification with no word highlights — technically a modification because the number changed, but the text is unchanged.

Test plan

• bun test packages/ui/utils/planDiffEngine.test.ts — all 18 tests pass
• bun run dev:hook — default demo (Real-time Collaboration) loads; toggle diff view; verify word-level highlights on the Context paragraph and code block
• VITE_DIFF_DEMO=1 bun run dev:hook — 20-case stress test loads; walk through each case and confirm behavior matches its "What to watch for" label
• Click "Classic" in the mode switcher — modified blocks render as the legacy stacked fallback
• Click "Raw" — git-style view unchanged
• Click a modified block → comment popover opens directly in selection mode (no hover-then-toolbar needed)
• Verify annotation on a modified block captures both old and new content in the exported feedback
• Build the hook bundle (bun run --cwd apps/review build && bun run build:hook) and smoke-test the single-file HTML