feat(search): show live match count and active match position

[le17603454419-collab]

Summary / 摘要

Motivation / 背景与动机

• Issue:
• Roadmap (docs/ROADMAP.md):
• OpenSpec change:

Changes / 变更内容

• packages/core:
• packages/plugin-*:
• apps/electron-demo:
• openspec/:

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：
• 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：
  
• 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)

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