Implements issue #14. A dedicated thread owns SDL3 (3.4.16, statically
linked via sdl3 =0.20.0) and publishes per-slot snapshots re-expressed as
the XInput-shaped XboxState, so the TU3 conversion in skate-core is
unchanged. Conversions are exact for XInput-backed pads (stick Y via
bitwise NOT, trigger round-to-nearest). SKATE3_INPUT=xinput selects the
original transport, which is also the fallback if SDL fails to start.
Optional settings/controller.json maps paddles to XInput buttons.
Build: scripts/Ensure-CMake.ps1 locates CMake (PATH or Visual Studio's
copy) for the SDL source build. sdl3-sys is vendored with a build.rs
patch setting CMAKE_MSVC_RUNTIME_LIBRARY from crt-static, because SDL's
CMake (policy CMP0091) ignores cmake-rs' -MT and the static-CRT release
build failed with unresolved __imp_* symbols.
See docs/hails-additions/03-sdl3-gamepad-input.md and vendor/README.md.
Hails