Chromium plugin: iframe support

[asklar]

Summary

Walk DOM across iframes (separate DOM per frame); today only the top frame is walked (docs/chromium-plugin.md → Future work).

Scope

• Enumerate frames via CDP (Page.getFrameTree) and walk each frame's DOM.
• Graft each child frame's DOM under its owning <iframe> element.
• Handle cross-origin/out-of-process iframes (OOPIFs) that have separate CDP targets.
• Update docs/chromium-plugin.md.

Acceptance criteria

• A page with same-origin and cross-origin iframes produces a complete, nested DOM.
• Tests covering frame enumeration/grafting.

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Filed as part of turning README/docs "Future work" into tracked issues. See docs/architecture.md and CONTRIBUTING.md (""Adding a new provider"") for conventions.