feat(core): add list item drag reorder and table row height resize

[wxd-hash]

Summary

Three enhancements to the live-preview engine:

1. List item drag reorder — hover to reveal ⋮ grip, drag to reorder, release to update Markdown source (OpenSpec §2.4)

2. Table row height resize — hover the bottom edge of any table row, drag to adjust row height

3. Fix table add-column bug — previously corrupted the table; now correctly inserts a new column

Motivation

• Roadmap: add-missing-features tasks.md §2.4 (list item drag reorder, P1)
• OpenSpec: openspec/changes/add-missing-features/ (already approved)
• Table already had column resize but no row height resize
• Table add-column button was broken with certain separator formats

Changes

• packages/core/src/live-preview.ts (+228 lines):
  List grip rendering (⋮), custom mousedown/mousemove/mouseup drag, editing lock (listEditingCount), CM6 coordsAtPos positioning, single-dispatch moveListLines, findItemEnd boundary detection
• packages/core/src/live-preview-table.ts (+103 lines):
  Row height resize (session-scoped tableRowHeights map, applyRowHeights/measureRowHeights/startRowResize, resize handle on first cell), addColumn rewrite (cell-split detection instead of SEPARATOR_RE anchor check)
• packages/core/test/live-preview.test.ts (+67 lines):
  3 tests: grip rendering, grip hidden when cursor on item, document reorder via setDocument

Testing

• pnpm test passes — 34 test files, 518 tests
• Affected packages build (pnpm build)
• New vitest cases: 3 (grip render, cursor hides grip, document reorder)
• Manual check in electron-demo: list drag reorder, row resize, add column

Compliance

• CLA signed — will sign when prompted by CLA bot
• AI disclosure: AI pair-programming used for implementation; all code reviewed and tested.
• New dependencies: none
• No build artifacts committed
• No secrets committed

Checklist

• Title follows Conventional Commits
• Public API changes update package README / types — N/A
• Touched live-preview-table.ts — walked through 12 Table Widget rules in CLAUDE.md
• New capability → OpenSpec proposal linked (openspec/changes/add-missing-features/)
• Change aligns with project scope (GOVERNANCE.md §4)

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