feat(input): physical game-controller support (Xbox / gamepads / joysticks)

[relmer]

Summary

Apple II joystick/paddle input is currently synthesized from the keyboard (arrows → joystick) and mouse (paddle/mouse) via the split input model + device selector. Add support for physical game controllers — Xbox controllers, generic USB gamepads, and joysticks — mapped onto the Apple II analog stick.

Mapping target

The Apple II game port is 2 analog axes + 2 buttons:

• Left stick X/Y → PDL0 / PDL1 (paddle timing at $C064/$C065, 0–255).
• Two face buttons → PB0 / PB1 ($C061 / $C062), same targets Open/Solid-Apple already drive.

Reading the device

DirectInput is a fine fit for this use case: mapping to the Apple II stick only needs the left-stick axes + two face buttons, all of which DirectInput exposes for both Xbox-class and generic controllers. (The one well-known DirectInput gotcha — Xbox triggers collapsed onto a shared Z axis — doesn't matter here, since the Apple II game port has no triggers.) XInput is worth revisiting later only if we want proper Xbox extras (correct triggers, Guide button, rumble); not required for v1.

Scope

• Enumerate controllers; hot-plug connect/disconnect detection.
• Poll the selected device each frame; axes → paddle values, buttons → PB0/PB1.
• Deadzone + axis→paddle scaling / centering calibration.
• Surface it in the existing device selector (a physical controller becomes the "joystick" source, coexisting with / overriding arrows→joystick).
• Persist the selection via the existing input-mapping prefs.

Notes

Fits the existing input model (Keys / Pointer split) — it's a new source feeding the existing paddle/button state, no new game-port hardware model needed.