A page on http(s) could set window.onload, assign location.href to a
file:// URL, and then read document.body from the local file because
two things lined up: obscura-net allows file: URLs, and the JS
runtime was reused across navigations so window state from the prior
origin survived into the new document's load event.
Two fixes:
1. Refuse a JS-initiated chain step when it crosses from a non-file
scheme into file:. Direct file:// navigations from the CLI/CDP
still work because the gate only fires on the take_pending_nav
path. file -> file chains also still work.
2. Drop and rebuild the V8 runtime on every navigation instead of
only rebinding globalThis.document. Even within the same origin
we should not carry over window.onload, custom properties, or
registered event listeners from the previous document.
Tested with the PoC from the report: pages on http://127.0.0.1.nip.io
no longer leak the contents of file:///tmp/.../pwn.txt.
