fix: ignore no-op stack resize grabs

[philip-sterne]

Summary

• Calculate resize movement from the window frame before adding stack-tab geometry.
• Correct the unchanged-height comparison in Movement.calculate().
• Stop a zero-motion resize grab before it can change tiling-tree ratios.

Root cause

Pop Shell records the raw window frame when a resize grab begins. For stacked windows, the grab-end path added the tab strip to the ending rectangle before comparing it with that raw starting frame. An edge click with no actual resize therefore appeared to be an upward grow operation.

The no-movement branch also compared the starting height with the ending width. Even when Movement.NONE was returned, it was accepted as valid and reached forest.resize(), where it followed the grow path.

Impact

Clicking a stack's resize edge without dragging no longer shrinks the stack or reflows neighboring tiled windows. Actual directional resize operations retain their existing behavior.

This resize-grab path is independent of #1824's DING desktop-window detection and focus handling.

Testing

• make compile
• Focused Movement.calculate() checks for unchanged, upward-grow, and downward-grow rectangles.
• Logged out and back in on Ubuntu with GNOME 50 and exercised the stack layout during normal use.