Nested NSHostingController layers (from bonsplit's SinglePaneWrapper)
prevent AppKit's NSDraggingDestination routing from reaching terminal
views. Install a transparent FileDropOverlayView on the window's theme
frame that intercepts file drags and forwards drops to the GhosttyNSView
under the cursor. Mouse events pass through via a hide-send-unhide
pattern.
Fix y-axis inversion in split targeting: hitTest expects coordinates in
the receiver's superview's coordinate system, not the receiver's own.
Converting to contentView's coords flipped y because NSHostingView is
flipped, causing top/bottom split drops to land in the wrong terminal.
Also adds bonsplit onFileDrop API, PaneDragContainerView, and
drop_hit_test socket command for testing coordinate-to-terminal mapping.
