* fix(browser): close guest-owned split tab
* fix: check sourceId before toggling floating panel on close
The empty-panel toggle is the ambient fallback only. Guest-initiated
closes (with sourceId) target the main workspace and should not toggle
the panel.
* test(browser-split-shortcuts): remove terminal-mirrors close test and un
Removes test case that verified Cmd+W closes guest-owned browser splits when
active-tab mirrors point to a terminal, along with the helper function and
unused fixture properties that only that test required.
