Slider controls now use the Pointer Lock API for drag interactions. When dragging a slider label, the cursor locks and movement continues infinitely past the screen edge, matching Unity3D and other professional 3D editors.
- requestPointerLock on drag start, movementX accumulation, exitPointerLock on release
- Falls back to setPointerCapture if PointerLock unavailable
- ESC during drag handled via pointerlockchange listener
- Shift/Ctrl step multipliers preserved
- Undo/redo temporal state correctly managed on all exit paths
Closes#204
