Fix iPad scroll source and natural direction

[alantangok]

Summary

• make GCMouse the sole scroll owner while a usable GCMouse source is connected; use UIKit pan only as a no-mouse fallback
• preserve the existing GameController axis mapping (yAxis -> wheelX, xAxis -> wheelY) and Natural Scrolling sign
• accumulate fractional GameController deltas without adding synthetic glide to discrete mouse-wheel input
• apply the existing Scroll Speed setting consistently to GameController and UIKit paths

Verification

• unsigned Release iOS device build succeeded
• Xcode Release static analysis succeeded
• exact-source mapper/accumulator checks cover axis mapping, Natural Scrolling direction, fractional accumulation, source ownership, and scroll sensitivity
• no project file, .checkpoint, or scripts/ changes

Device validation still required

• Magic Keyboard trackpad: smooth vertical/horizontal scrolling with Natural Scrolling on and off
• physical discrete mouse wheel: one notch must not receive synthetic momentum
• disconnect/reconnect the mouse and confirm UIKit fallback activates without duplicate events

Split from #93.