Add 60fps cutscene clamp for in-engine cinematics

Suspend the FPS unlock while a demo-manager Exec (DD sub_82184460 / EM sub_821856F8) ticks, so the frame-locked IngameCinematics Sequencer plays at native ~30fps instead of double speed. Adds ac6_cutscene_clamp CVar (default on).

src/render_hooks.h

@@ -7,6 +7,7 @@
 
 REXCVAR_DECLARE(bool, ac6_unlock_fps);
 REXCVAR_DECLARE(bool, ac6_timing_hooks_enabled);
+REXCVAR_DECLARE(bool, ac6_cutscene_clamp);
 
 namespace ac6 {
 
@@ -18,9 +19,18 @@ struct FrameStats {
 
 FrameStats GetFrameStats();
 
+// 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.
+bool IsCinematicActive();
+
 }  // namespace ac6
 
 bool ac6FlipIntervalHook();
 bool ac6PresentIntervalHook(PPCRegister& r10);
 void ac6DeltaDivisorHook(PPCRegister& r29);
 void ac6PresentTimingHook(PPCRegister& r31);
+
+// Fires once per frame from the demo-manager Exec while a cutscene is playing.
+// r3 = the demo-manager `this`; unused (presence-of-call is the signal).
+void ac6CinematicTickHook(PPCRegister& r3);