fix(toolbar): isolate consecutive command history

[sssssirius7]

Summary / 摘要

Motivation / 背景与动机

• Issue: #283
• Roadmap (docs/ROADMAP.md): N/A — bug fix for existing Toolbar commands; no new roadmap item.
• OpenSpec change: Pending — this PR adds the opt-in public replaceRange history-isolation option.

Changes / 变更内容

• packages/core:

  • Add the opt-in isolateHistory option to EditorAPI.replaceRange().
  • Use CodeMirror's full history isolation only when the option is enabled.
  • Clarify replacement and selection coordinate semantics in the public API docs.
  • Add core regression coverage for consecutive isolated edits.

• packages/plugin-*:

  • Migrate Toolbar editing commands from setDocument() + setSelection() to atomic replaceRange().
  • Enable isolated history entries for consecutive Toolbar operations.
  • Cover formatting commands, links, headings, blockquotes, lists, code blocks, images, colors, highlights, horizontal rules, and the Normal text menu action.
  • Add regression tests for selections, undo behavior, consecutive commands, and IME composition flushing.

• apps/electron-demo:

  • No changes.

• openspec/:

  • No proposal added yet; pending maintainer guidance for the new optional public API.

Testing / 测试

• pnpm test passes / 全绿
  未运行全量测试；已运行受影响测试，共 97 个用例通过。
• Affected packages build (pnpm build) / 受影响包构建通过
  @floatboat/nexus-core和@floatboat/nexus-plugin-toolbar` 构建通过。
• New / updated vitest cases / 新增或更新的 vitest 用例：
  Added coverage for atomic replacement, isolated Toolbar history, exact selections, consecutive Toolbar undo, Normal text UI action, and IME composition flushing.
• Manual UI check in electron-demo / electron-demo 手动验证：

Compliance / 合规自检

• CLA signed — first-time contributors will be prompted automatically by the CLA bot / 首次贡献者按 CLA 机器人提示签署
• AI disclosure: the functional code in this PR is not primarily generated by AI. AI assistance, if any, is described below.
  AI-assisted notes / AI 使用说明：
  
  AI tools assisted with code exploration, implementation, test authoring, and review.
• New dependencies (if any) listed with license & rationale (none if blank):
  
• 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 — 改了公共 API 已同步 README 与类型
• Touched live-preview-table.ts → walked through the 12 Table Widget rules in CLAUDE.md / 已核对 12 条表格规则
• New capability / breaking change → OpenSpec proposal linked / 新 capability 或破坏性变更已附 OpenSpec
• Change aligns with project scope (GOVERNANCE.md §4) / 改动符合 GOVERNANCE.md §4 的项目范围

Screenshots / Recordings · 截图或录屏 (UI changes)

[CLAassistant]

All committers have signed the CLA.