handleForwardCdpCommand fell through to anyConnectedTab() when a
daemon-supplied sessionId didn't map to an attached tab, so eval/screenshot/
network silently ran on an arbitrary tab — the root of "eval ran on the
wrong page, no warning" and the blank-screenshot-after-restart symptom.
Now: a provided sessionId/targetId MUST resolve to a real tab or the command
throws an actionable error ("stale sessionId … re-open your target URL").
anyConnectedTab() is only used for genuinely browser-level commands that
specify neither. Manifest 0.4.1 → 0.4.2 (needs a Chrome Web Store republish
for installed users to pick this up).