fix: skip Stepwise runtime when disabled

[DamienZCollins]

Summary

• Include the Stepwise runtime in the new-document injection bundle only when codexAppStepwiseEnabled is enabled.
• Stop an already injected runtime from recreating its floating UI, observer, or generation request after the setting is disabled.
• Clarify in the manager and injected settings panel that changing the Stepwise setting requires restarting Codex++.
• Add disabled and enabled bundle regression coverage.

Validation

• cargo fmt --check
• cargo test -p codex-plus-core --test cdp_bridge injection_script_
• cargo test -p codex-plus-core --test cdp_bridge stepwise_
• npm run check in apps/codex-plus-manager
• cargo build -p codex-plus-launcher --release

Notes

The full cdp_bridge suite ran 80/81 tests successfully; its IPv6-loopback integration test fails in this Windows environment with socket access denied, independently of this change. node tools/i18n-verify.mjs still reports 13 pre-existing, unrelated VLM translation keys; the new Stepwise key is not among them.

Fixes #1549

[BigPizzaV3]

维护者审查：当前改动的功能逻辑没有发现 blocker。disabled bundle 不包含 Stepwise runtime，enabled 时在 ensureSettings() 后激活，运行中关闭也会走 stopRuntime()，与 UI 的“重启生效”提示一致。

但分支目前与 main 冲突，且没有任何 CI 结果。请 rebase/解决冲突后触发三平台 checks，并确认相关 cdp_bridge tests 通过；通过后可以合并。

[BigPizzaV3]

Codex++ maintainer review note

当前 main 下这个 PR 仍存在 merge conflict，暂时不能合并。请先 rebase 到最新 main 并解决冲突；更新后我们会重新跑针对性测试再复审。

This PR currently conflicts with main, so it is not mergeable yet. Please rebase onto the latest main and resolve the conflicts, then we can review and test it again.