feat(html): implement HTMLIFrameElement contentWindow/contentDocument

[Sonic-Y3k]

HTMLIFrameElement: contentWindow / contentDocument

Adds an HTMLIFrameElement exposing the nested browsing context via contentWindow
and contentDocument, and creates iframe elements from
document.createElement("iframe").

Compliance / known limitation

A faithful implementation would give each iframe its own same-origin JavaScript
realm (distinct native built-ins, mutually accessible with the parent). gost-dom
binds one realm per script context, and the v8go build does not expose
security-token control, so a separate child context's globals are not
cross-realm accessible. Until that lands, the nested context resolves to the
owning window's realm: every native built-in on contentWindow (Function, String,
eval, ...) stays accessible and genuine; only object identity differs from a
real same-origin iframe.

Split out of #274. The script-engine binding lives in its own file and the test
runs in the shared suite, so both the V8 and Sobek engines are covered.

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AI disclosure: This change was developed with the help of an AI coding assistant. I've reviewed and tested it myself; it follows the existing conventions and the full test suite (main module, v8engine, sobekengine) passes locally.

[coderabbitai]

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ℹ️ Review info

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Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: b3de96ba-e16b-4cba-9f0e-70fb9bd8a1d2

📥 Commits

Reviewing files that changed from the base of the PR and between 98adcf9 and c8854d1.

📒 Files selected for processing (6)

• html/html_document.go
• html/html_iframe_element.go
• scripting/internal/html/html_iframe_element.go
• scripting/internal/html/initializer.go
• scripting/internal/scripttests/iframe_suite.go
• scripting/internal/scripttests/suites.go

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