refactor: rename Controller* events to Gamepad* for SDL3 consistency

[revmischa]

Renames event types and variants to match SDL3's gamepad terminology:

Old Name	New Name
ControllerAxisMotion	GamepadAxisMotion
ControllerButtonDown	GamepadButtonDown
ControllerButtonUp	GamepadButtonUp
ControllerDeviceAdded	GamepadAdded
ControllerDeviceRemoved	GamepadRemoved
ControllerDeviceRemapped	GamepadRemapped
ControllerTouchpadDown	GamepadTouchpadDown
ControllerTouchpadMotion	GamepadTouchpadMotion
ControllerTouchpadUp	GamepadTouchpadUp
ControllerSensorUpdated	GamepadSensorUpdated

Also renames is_controller() to is_gamepad() with a deprecated alias for backward compatibility.

Breaking change: Event variant names changed from Controller* to Gamepad*. The deprecated is_controller() alias helps migration.

This PR depends on #322 and together they complete #319.

[Copilot]

Pull request overview

This PR aligns the Rust event API with SDL3’s “gamepad” terminology by renaming Controller* event types/variants to Gamepad*, and introduces is_gamepad() (with a deprecated is_controller() alias) to ease migration. It also includes the JoystickId newtype work (per dependency on #322) and updates call sites/examples accordingly.

Changes:

• Rename EventType::Controller* and Event::Controller* variants to Gamepad* equivalents.
• Rename Event::is_controller() to Event::is_gamepad() and keep is_controller() as a deprecated alias.
• Convert joystick/gamepad which identifiers to use the JoystickId newtype and update subsystem bindings/examples.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/sdl3/joystick.rs	Introduces JoystickId as a transparent newtype and updates joystick APIs to convert into SDL’s ID type.
src/sdl3/gamepad.rs	Updates gamepad subsystem APIs to accept JoystickId and pass the underlying SDL ID via .into().
src/sdl3/event.rs	Renames controller events/types to gamepad equivalents; adds is_gamepad() and deprecated is_controller() alias.
examples/sensors.rs	Updates sensor event matching to Event::GamepadSensorUpdated.
examples/gamepad.rs	Updates event matching arms to new Gamepad* event variants.

Comments suppressed due to low confidence (1)

src/sdl3/event.rs:821

• These doc comments still refer to "controller" even though the variants were renamed to Gamepad*. Updating the wording keeps the public API docs consistent with the SDL3 gamepad terminology.

    GamepadAxisMotion {
        timestamp: u64,
        /// The controller's joystick `id`
        which: JoystickId,
        axis: Axis,

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