Add ControllersToGamepadRetargeter

[rwiltz]

Description

Maps a pair of XR controllers (ControllersSource) to gamepad_buttons and gamepad_axes in the standard gamepad layout, so the gamepad retargeters (SE3, SE2, gripper) drive a robot from controllers
unchanged.

Mapping

• Thumbsticks → LEFT_X/Y, RIGHT_X/Y (OpenXR's up-positive Y passes straight through)
• Triggers → LEFT/RIGHT_TRIGGER; grips → LEFT/RIGHT_SHOULDER (above grip_threshold)
• A / B (right) → SOUTH / EAST; X / Y (left) → WEST / NORTH; thumbstick clicks → LEFT/RIGHT_STICK; left menu → START
• Controllers have no D-pad: dpad_stick="left"|"right" makes that thumbstick the D-pad while its grip is held (enables SE3 roll/pitch). Off by default.
• A missing controller reads as released and centered; outputs are absent only while both are.

The gamepad printer example gains --source controllers (and --dpad-stick) to try it from a headset.

Fixes #(issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Testing

Testing

• 8 unit tests with synthetic controller input, including an end-to-end gripper toggle on left X.
• Tested on a Quest 3 through CloudXR with the printer example.

Checklist

• I have read and understood the contribution guidelines
• I have run the linter and formatter with SKIP=check-copyright-year pre-commit run --all-files
• I have made corresponding changes to the documentation
• I have added tests that prove my fix/feature works (or explained why not)
• I have signed off all my commits (git commit -s) per the DCO

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