Document and test extension integration

[MiguelsPizza]

WIP - currently having some AI's do reviews. Below is AI generated

this adds a guide for dropping the polyfill into an existing extension, on top of #2. It covers installing the built bundle as a MAIN-world content script at document_start, discovering and invoking through scripting.executeScript so the browser carries results back without a page-message protocol, and forwarding toolchange into the isolated world as a payload-free DOM event. The guide is explicit that those events are hints and never an authorization boundary, since MAIN-world code shares the page's globals.

The test lifts the guide's two JavaScript blocks, wraps them in an MV3 extension, and runs it against a real served page in Chromium and in stock Firefox over geckodriver, covering discovery, execution, unregister and re-register, reload, and a non-matching origin. Firefox runs the same chrome.scripting code through Selenium, because Playwright loads extensions only in Chromium. Extraction depends on the guide having exactly two js blocks in that order plus two literal markers, so the test asserts the extraction itself: editing EXTENSIONS.md fails in a second with a message naming the file instead of timing out somewhere inside a 90-second extension run.

All 102 browser tests and the package checks pass; WPT is unchanged from #2, same five expected failures. That closes the stack.