Function pane_terminal_resolve_splitter_target 

pub fn pane_terminal_resolve_splitter_target(
    handles: &[PaneTerminalSplitterHandle],
    x: u16,
    y: u16,
) -> Option<PaneResizeTarget>

Resolve a semantic splitter target from a terminal cell position.

If multiple handles overlap, chooses deterministically by:

1. smallest distance to the splitter boundary, then
2. smaller split_id, then
3. horizontal axis before vertical axis.