input: add accessibility co-pilot joypad feature

[Iansboro]

Closes #7830 (N-to-1 direction)

Inspiration

This feature is inspired by the Xbox Copilot accessibility feature,
which allows a second controller to assist a player with disabilities.
Unlike the existing "Mapped Port" workaround (which is not easily discoverable), this implementation provides
a dedicated, persistent, globally-applicable setting directly in the
Port N Controls menu where users already configure their controllers.

What this does

Allows a second physical controller ("co-pilot") to share input with
an existing player port. Intended for users with disabilities who need
assistance from a caregiver or companion controller.
Persistent: configured once in retroarch.cfg, applies to all cores and games automatically.

How it works

• Settings → Input → Port N Controls → Co-pilot Joypad
• Digital buttons: OR'd together (either pad can press any button)
• Analog axes: max-magnitude merge (whichever pad is pushed further wins)
• Hotkeys: the co-pilot's own autoconf profile is used, so a different
  controller model can trigger basic hotkeys (e.g. Start+Select for menu)

Tested on

• Raspberry Pi 5 (aarch64, Linux)
• Main pad: Xbox 360 USB
• Co-pilot pad: Mad Catz Street Fighter IV FightPad
• Core: Genesis Plus GX (Sonic the Hedgehog)
• Both pads successfully controlled Player 1 simultaneously

Known Limitations

• Complex hotkey combinations (e.g. Select+R1 for save state) require
  the co-pilot pad to have the same physical button layout as the main
  controller. Basic hotkeys (Start+Select for menu toggle) work correctly
  with any controller model. Will be addressed in a follow-up PR.

Files changed

• configuration.h — COPILOT_PORT_DISABLED constant + input_copilot_port[]
• configuration.c — default init + .cfg persistence (p1–p8)
• input/input_driver.c — game input merge + hotkey merge
• menu/menu_setting.c — UI entry per player port with navigation callbacks

Accessibility

This feature has no impact on users who do not configure it
(default = COPILOT_PORT_DISABLED for all ports).

[gouchi]

Hi,

I tested quickly and it is working.

You got a new parameter

[Iansboro]

Hi,

I tested quickly and it is working.

You got a new parameter

Thank you for testing! Great to see it working with two different controller models (Xbox 360 + PS4).