chore: bump version to 1.9.1

[AmisKwok]

Summary / 摘要

Bumps the package version to 1.9.1 for the packaging cleanup that landed after v1.9.0 was published. / 将包版本提升至 1.9.1，用于发布 v1.9.0 之后合入的发布包精简改动。

Changes / 变更

• pubspec.yaml: version: 1.9.0 → 1.9.1. / 版本号提升至 1.9.1。
• ios/zero_inspector_kit.podspec: s.version → '1.9.1'. / iOS podspec 版本同步为 1.9.1。
• README.md / README_zh.md: dependency constraint ^1.9.1, GitHub install ref: release/v1.9.1, and the "Upgrade recommended" callout now summarizes the smaller published archive. / 安装约束 ^1.9.1、GitHub 安装 ref: release/v1.9.1，升级提示改为说明发布包更小。
• CHANGELOG.md: new ## 1.9.1 section describing the trimmed published package (~23 MB → ~3 MB, no runtime change). / 新增 ## 1.9.1 条目，仅描述发布包体积优化（约 23 MB 降至约 3 MB，无运行时变化）。

Context / 背景

v1.9.0 was already published to pub.dev on 2026-09-06 and a published version cannot be overwritten, so the packaging improvements merged afterwards ship as a patch release 1.9.1. No lib/ runtime behavior changed. / v1.9.0 已于 2026-09-06 发布到 pub.dev，已发布版本不可覆盖，因此之后合入的发布包改动以 1.9.1 补丁版本发布。lib/ 运行时行为无变化。

Checklist / 检查项

• Title follows Conventional Commits / 标题符合约定式提交
• CI checks pass after merge / 合入后 CI 通过

Test plan

• flutter pub publish --dry-run reports version 1.9.1 with Package has 0 warnings. / dry-run 显示 1.9.1 且 0 warnings
• README.md / README_zh.md show ^1.9.1 and ref: release/v1.9.1 / 两个 README 均显示 ^1.9.1 与 ref: release/v1.9.1
• After merge: tag v1.9.1 on the merge commit and archive branch release/v1.9.1. / 合并后在合并提交上打 v1.9.1 tag 并建 release/v1.9.1 归档分支。

🤖 Generated with Zero Buddy