Add cross-platform gamepad rumble via SDL2

[chiefautism]

Summary

• New crate: buttplug_server_hwmgr_sdl_gamepad — cross-platform gamepad haptics via SDL2
• Xbox, PlayStation, and Switch controllers appear as XInput-compatible buttplug devices with dual vibration motors (left/right, 65535 steps each)
• Works on macOS (GCController backend), Windows (XInput/DirectInput), Linux (evdev)
• New CLI flag: intiface-engine --use-sdl-gamepad
• SDL2 statically linked via bundled feature — no system dependency required

Motivation

buttplug currently supports gamepads only via XInput on Windows. SDL2 provides a single cross-platform API for gamepad rumble that works everywhere, including macOS where XInput is unavailable.

This unlocks game controllers as haptic devices for creative applications — interactive fiction with haptic feedback, biofeedback loops, accessibility tools, and more.

Implementation

• SdlGamepadCommunicationManager: scans for controllers via SDL2's GameController API on a dedicated thread (SDL types are !Send)
• SdlGamepadHardware: wraps a GameController with a worker thread that processes rumble commands, with 5ms drain batching to coalesce per-motor write_value calls
• Reuses the existing XInput protocol/device config — gamepads appear as "XBox (XInput) Compatible Gamepad"
• Stop fix: SDL on macOS requires duration > 0 for zero-rumble to override active rumble

Test plan

• Build on macOS ARM64 (Apple Silicon)
• Xbox Series X controller detected via Bluetooth
• Vibrate at various intensities (ScalarCmd)
• Stop working correctly (StopDeviceCmd)
• Build on Windows
• Build on Linux
• Test with DualSense
• Test with Switch Pro Controller

🤖 Generated with Claude Code

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[qdot]

Oh hell yeah. Lemme know what platforms you need tests run on if you don't have them yourself. I've also got xbox/dualsense/pro controllers to test with.

[qdot]

Ok, so, you changed the copyright on this and haven't signed the contributor agreement, so I can't bring this in until those are done. Looking pretty decent otherwise, there's a race condition on discovery and it doesn't seem to detect/emit on devices being disconnected, but I can write those up in a review after the licensing stuff gets done.