Function handle_drain_browser_pane_by_label 

pub(super) fn handle_drain_browser_pane_by_label(
    state: &mut HostState,
    label: String,
) -> DispatchOutput

PR #5 — sole label-keyed drain entry point. Replaces pane::lifecycle::PaneStateMachine::drain_by_label.

Removes whichever pane entry has label. Returns the drained block_id in drained_browser_pane_block_id. Idempotent — None if no entry has that label (e.g., explicit close() already cleared it; on_before_close arrives later).