The on_request/on_response Vec lived on the ObscuraHttpClient shared by
every page in the context, so a callback registered on one page fired for
all pages (#408 cross-page bleed) and outlived its page.
Callbacks now live in a per-page CallbackRegistry (Arc shared with the
page's JS runtime state). Page-driven fetch paths (navigation, POST,
scripts, stylesheets, robots.txt, module loads, JS fetch/XHR in both
plain and stealth modes) pass the registry through; the HTTP client
itself is callback-free.
Adds callbacks_do_not_bleed_across_pages: page A's callback stays silent
for page B's requests and stops firing after page A is dropped.
Jim Zhang @ wonderhoi
f5dc97b131