Restrict local action URL validation to form elements

[chubes4]

Problem

WordPressSitePlan::assertNoLocalBrowserReferences() validates every HTML action attribute as a navigable browser URL, regardless of element name. Custom elements can legitimately use action as an enum.

A direct import of https://www.tasteandtravelitaly.com/blog.html contains <fb:like action="like">. The plan validator interprets like as an unresolved local form action and aborts compilation with unresolved_local_browser_reference.

This is a false positive. The action URL contract applies to <form>; the custom element attribute is Facebook widget metadata.

Expected contract

• Validate action as a browser reference only on <form> elements.
• Continue validating src, href, poster, srcset, style URLs, and serialized block URL fields as today.
• Preserve fail-closed validation for unresolved local <form action> references established by Canonicalize local form actions through the WordPress route map #672.
• Add deterministic coverage for both the custom-element false positive and a real unresolved form action.

Tracker integration: Automattic/studio#3952 and Automattic/static-site-importer#895.

AI assistance

OpenAI gpt-5.6-sol via OpenCode reproduced the direct import failure, traced the plan validator, and drafted this issue. Chris Huber remains responsible for the report and resulting changes.