Add native KB+M support: menus, flight, mouse steering, pad-free play

Keyboard/mouse state is injected at the guest XamInput wrapper boundary, so
every layer of the input stack sees KB+M exactly as a connected pad, and a
real pad keeps working alongside. Menu and flight key sets (AC7-style
defaults) switch automatically via a flight-sim heartbeat; the mouse steers
pitch/roll with a velocity model, or pans the camera while the camera-control
key is held. With no controller connected, a synthetic pad is presented on
slot 0. Bindings and mouse feel live in ac6_input.toml (auto-created on first
run, hot-reloaded ~1x/s). Everything is off unless ac6_kbm_enabled is set.

CMakeLists.txt

@@ -41,6 +41,7 @@ set(AC6RECOMP_SOURCES
     src/ac6_backend_fixes/ac6_backend_hooks.cpp
     src/ac6_backend_fixes/ac6_backend_pass_classifier.cpp
     src/ac6_backend_fixes/ac6_fps_physics_fix.cpp
+    src/ac6_backend_fixes/ac6_kbm_input.cpp
 )
 
 if(WIN32)

src/ac6_backend_fixes/ac6_fps_physics_fix.cpp

@@ -164,11 +164,18 @@ void BlendFieldDelta(uint8_t* base, uint32_t ea, float pre, double ratio, const
 PPC_EXTERN_FUNC(__imp__rex_sub_823046A0);  // flight-model force step
 PPC_EXTERN_FUNC(__imp__rex_sub_82329B40);  // flight-model input shaping
 
+// KB+M context switch (ac6_kbm_input.cpp): the force step is the reliable
+// "flight sim is stepping" heartbeat (runs per frame for every aircraft,
+// halts when paused / in menus).
+void ac6KbmNotifyFlightStep();
+
 // Flight-model core force step (0x823046A0): rescale the per-frame stepped
 // longitudinal force/speed command at [this+1320].
 PPC_FUNC_IMPL(rex_sub_823046A0) {
   PPC_FUNC_PROLOGUE();
 
+  ac6KbmNotifyFlightStep();
+
   const uint32_t self = ctx.r3.u32;
   const double ratio = StepRatio();
   // 1.0 is the exact pass-through value (native cadence, or the fix disabled).