remove_page_by_target_id left active_target_id dangling when the pinned
page itself was removed, so resolved_active_index silently fell back to
active_page_index — which after a passive about:blank discovery can point
at a blank tab. That matches issue #7's intermittent symptom: `wait` then
eval/snapshot landing on about:blank in a --launch session.
Re-pin to the surviving active page after removing the pinned target so
the pin is never left pointing at a target that no longer exists. Adds
pure regression tests for the re-anchor invariant (BrowserManager needs a
live CDP client, so the method can't be unit-constructed directly).
leeguooooo
0e5409a81e