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.cpp

@@ -11,6 +11,9 @@
 REXCVAR_DEFINE_BOOL(ac6_unlock_fps, false, "AC6", "Unlock frame rate to 60fps");
 REXCVAR_DEFINE_BOOL(ac6_timing_hooks_enabled, true, "AC6",
                     "Enable AC6 timing hooks that alter the game's presentation cadence");
+REXCVAR_DEFINE_BOOL(ac6_cutscene_clamp, true, "AC6",
+                    "Suspend the 60fps unlock during in-engine cutscenes so they "
+                    "play at native ~30fps instead of double speed");
 
 using Clock = std::chrono::steady_clock;
 
@@ -21,12 +24,44 @@ std::atomic<double> g_fps{0.0};
 std::atomic<uint64_t> g_frame_count{0};
 Clock::time_point g_frame_start{};
 
+// Wall-clock (steady) ms of the last in-engine cutscene tick. The cutscene
+// hook (ac6CinematicTickHook) runs on the game thread; the timing hooks read
+// this on the present/GPU path, hence the atomic. INT64_MIN = never ticked.
+std::atomic<int64_t> g_last_cinematic_tick_ms{INT64_MIN};
+
+// The demo-manager Exec runs every game frame while a cutscene plays (~16-33ms
+// apart). A few-frame decay keeps the clamp asserted across the cutscene and
+// auto-releases shortly after it ends.
+constexpr int64_t kCinematicDecayMs = 100;
+
+int64_t NowMs() {
+    return std::chrono::duration_cast<std::chrono::milliseconds>(
+               Clock::now().time_since_epoch())
+        .count();
+}
+
 bool AreTimingHooksActive() {
-    return REXCVAR_GET(ac6_timing_hooks_enabled) && REXCVAR_GET(ac6_unlock_fps);
+    return REXCVAR_GET(ac6_timing_hooks_enabled) && REXCVAR_GET(ac6_unlock_fps) &&
+           !ac6::IsCinematicActive();
 }
 
 }  // namespace
 
+namespace ac6 {
+
+bool IsCinematicActive() {
+    if (!REXCVAR_GET(ac6_cutscene_clamp)) {
+        return false;
+    }
+    const int64_t last = g_last_cinematic_tick_ms.load(std::memory_order_relaxed);
+    if (last == INT64_MIN) {
+        return false;
+    }
+    return (NowMs() - last) <= kCinematicDecayMs;
+}
+
+}  // namespace ac6
+
 bool ac6FlipIntervalHook() {
     return AreTimingHooksActive();
 }
@@ -61,6 +96,15 @@ void ac6PresentTimingHook(PPCRegister& /*r31*/) {
     g_frame_start = now;
 }
 
+void ac6CinematicTickHook(PPCRegister& /*r3*/) {
+    // A demo-manager Exec (DD: sub_82184460 / EM: sub_821856F8) ran this frame ->
+    // an in-engine cutscene is playing. Stamp the time so AreTimingHooksActive()
+    // suspends the 60fps unlock until the cutscene ends (the stamp goes stale a
+    // few frames after the last tick), letting the frame-locked cinematic
+    // Sequencer play at native ~30fps instead of double speed.
+    g_last_cinematic_tick_ms.store(NowMs(), std::memory_order_relaxed);
+}
+
 namespace ac6 {
 
 FrameStats GetFrameStats() {