ci(release): add macOS Intel build to release matrix

[Cmochance]

Summary

• Adds a third build matrix entry (macos-x64 on macos-13, target x86_64-apple-darwin) so each tag push produces _x64.dmg / _x64.pkg alongside the existing Apple Silicon and Windows assets.
• Release job now attaches the Intel artifacts to the draft GitHub release.
• README + README.zh-CN updated to list the new Intel assets and the expanded matrix; CHANGELOG entry deferred to the next version-bump PR (project convention: CHANGELOG sections are released versions only).

Why

Intel Mac users currently can't run the Apple Silicon _aarch64 builds without Rosetta + Tauri quirks; native x86_64 DMG/PKG closes that gap.

Notes

• Host-arch builds keep scripts/macos-artifacts.mjs + scripts/create-macos-pkg.mjs working as-is — no --target is passed; process.arch === 'x64' on the Intel runner yields _x64 suffixes that match Tauri's own dmg naming.
• macos-13 is the only Intel-runner option GitHub provides; it is still supported but has been flagged for eventual deprecation. Monitor actions/runner-images for a successor.

Test plan

• PR CI green on Linux (ci job) — pure YAML/markdown change should pass type-check / cargo lib check.
• After merge, push a throwaway vX.Y.Z-test tag (or a real v1.5.11) and confirm:
  • Three build jobs run (macos-arm64, macos-x64, windows-x86_64) and all upload artifacts.
  • Draft release contains _aarch64.dmg, _aarch64.pkg, _x64.dmg, _x64.pkg, _x64-setup.exe.
  • _x64.dmg opens on an Intel Mac (or via Rosetta-off VM) and the bundled .app launches.
• Reminder to add a CHANGELOG bullet ("first release with macOS Intel build") to the next version-bump PR.

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[Cmochance]

需要你确认 / 完成的事项

Merge 前

• 等 CI 全绿（Linux ci job 应该是 PR 上唯一会跑的;build matrix 仅 tag/dispatch 触发,所以 PR 上看不到 macOS Intel 实跑结果)。
• 如果想在 merge 前先验证 Intel 构建是否真能跑出包,可以在这个 PR 分支上手动触发一次 workflow_dispatch（Actions → Build → Run workflow → 选 feat/macos-intel-build 分支)。这次 dispatch 也会跑 macos-arm64 + macos-x64 + windows-x86_64 三个 job,产物只在 artifacts 里,不会发 release。

Merge 后第一次 push tag(下一次 release)

• Tag push 后到 Releases 页确认 draft 里五份产物齐全:_aarch64.dmg / _aarch64.pkg / _x64.dmg / _x64.pkg / _x64-setup.exe。
• 找一台 Intel Mac(或 macOS VM 关掉 Rosetta)挂载 _x64.dmg,确认 .app 能直接启动(本仓库 macOS 产物未做签名 / 公证,首次启动会被 Gatekeeper 拦,系统设置 → 隐私与安全里放行即可)。
• 在下一次 version-bump PR 的 CHANGELOG 里加一条 "first release with macOS Intel build" 之类的 bullet(此 PR 没碰 CHANGELOG,保持项目"每段都是已发布版本"的约定)。

风险提醒

• macos-13 runner GitHub 已经标了将来 deprecate,目前还能用;若哪天 runner 下线,需要切到当年 GitHub 官方推荐的 Intel 替代镜像(关注 actions/runner-images 仓库)。