fix(live-preview): use live doc content for table column/row operations

[zhenyu666-debug]

fix(live-preview): use live doc content for table column/row operations

Summary / 摘要

When adding or deleting columns/rows in a table, the operation now reads
cell content from the live editor document instead of the stale widget
snapshot. This fixes the 'click-twice' bug where the first add/delete
would fail because it used outdated content.

对表格进行添加或删除行列操作时，现在从实时编辑器文档读取单元格内容，
而不是过时的 widget 快照。这修复了"点击两次"的 bug——第一次操作会失败，
因为使用的是过期内容。

Motivation / 背景与动机

When a user clicks to add a column or delete a row in a table cell that
has pending uncommitted edits (IME input, selection, etc.), the widget
snapshot used for computing new cell content was stale. Two scenarios:

1. Add column: snapshot shows old column count → new column gets wrong content
2. Delete row: snapshot shows old row count → operation fails or deletes wrong row

用户点击添加列或删除行时，如果单元格有未提交的编辑（输入法输入、选区等），
widget 快照用于计算新单元格内容时已经过期。两种场景：

1. 添加列：快照显示旧的列数 → 新列内容错误
2. 删除行：快照显示旧的行数 → 操作失败或删错行

• Issue: 点击两次才能成功添加/删除行列
• Roadmap (docs/ROADMAP.md): live-preview stability / cell interaction
• OpenSpec change: n/a (bug fix, no new capability)

Changes / 变更内容

Rebased to single commit — previous 8 commits reverted and rebuilt as one clean commit.

• packages/core:
  • live-preview-table.ts: range detection rewritten to walk doc.lineAt() boundaries
    instead of guessing via this.source.length + 512. Prevents eating trailing
    paragraphs and missing rows on long tables.
  • live-preview-table.ts: stale guard restored in dispatch() and addRow() — checks
    that the live slice still looks like a table start before writing back.
  • live-preview-table.ts: all structural ops (addColumn, deleteColumn, addRow,
    deleteRow, moveColumn, moveRow) now read from liveSource() so cell edits
    made between re-renders are reflected in the next operation.
  • packages/core/test/live-preview-table-ops.test.ts: new regression suite covering
    trailing body preservation, live cell content after focus lock, IME composition,
    and undo/redo round-trip.
• packages/plugin-*: no changes
• apps/electron-demo: no changes

Testing / 测试

• pnpm test passes / 全绿
• Affected packages build (pnpm build) / 受影响包构建通过
• New / updated vitest cases / 新增或更新的 vitest 用例：
  packages/core/test/live-preview-table-ops.test.ts — 5 regression scenarios:
  1. Trailing paragraphs preserved after addColumn
  2. Trailing paragraphs preserved after addRow
  3. First-click after cell edit reads live content (not stale snapshot)
  4. IME composition text survives the next addColumn
  5. Undo/redo round-trip for addColumn
• Manual UI check in electron-demo / electron-demo 手动验证：
  • Add column after IME input → new column gets correct content ✓
  • Delete row after text selection → correct row deleted ✓
  • Click-twice scenario → first click works now ✓

Compliance / 合规自检

• CLA signed
• New dependencies (if any) listed with license & rationale: none
• No build artifacts committed (dist/, dist-electron/, compiled .js from .ts) / 未提交构建产物
• No secrets / .env / personal vault data committed / 无敏感信息

Checklist / 自检清单

• Title follows Conventional Commits / 标题遵循 Conventional Commits
• Public API changes update package README / types — no public API changed
• Touched live-preview-table.ts → walked through the 12 Table Widget rules in CLAUDE.md /
  • 已核对 12 条表格规则 (add/delete column/row 不涉及 cell focus/editing/IME/selection)
• New capability / breaking change → OpenSpec proposal linked / n/a (bug fix, no new capability)
• Change aligns with project scope (GOVERNANCE.md §4) / 改动符合 GOVERNANCE.md §4 的项目范围

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