Added modern present pacing + dt-snap: smooth fps unlock

Guest swap thread blocks on real GPU delivery (NotifyGuestPresent feeds the delivery counter) + an absolute-deadline limiter at ac6_fps_target, not the vblank grid (no sub-harmonic staircase). dt-snap locks the sim delta to the exact target at steady rate (kills the shake). Guest vblank forced to free-run during gameplay so pacing needs zero config. ac6_unlock_fps is the single master switch, ON by default (folded in timing-hooks/vblank-auto/delta-precision/dt-snap-tolerance AND the flight-model framerate-independence fix).

src/render_hooks.h

@@ -6,7 +6,6 @@
 #include <rex/ppc/types.h>
 
 REXCVAR_DECLARE(bool, ac6_unlock_fps);
-REXCVAR_DECLARE(bool, ac6_timing_hooks_enabled);
 REXCVAR_DECLARE(bool, ac6_cutscene_clamp);
 
 namespace ac6 {
@@ -24,6 +23,12 @@ FrameStats GetFrameStats();
 // off this so it stays in lockstep with the frame-delta hooks.
 bool TimingHooksActive();
 
+// ac6_min_sim_fps clamped to its supported range [10, 30]. Single source for
+// the low-fps floor shared by the frame-delta clamp (ac6DeltaPrecisionHook)
+// and the physics step cap (StepRatio) - the two must saturate at the same
+// framerate or the dynamics desync from the kinematics below 30fps.
+double ClampedMinSimFps();
+
 // True while an in-engine cutscene (NU::FW::IngameCinematics, driven by
 // CAce6DemoManager::Exec) has ticked within the last decay window. Used by the
 // timing hooks to suspend the 60fps unlock so cutscenes play at native cadence.