Video mirror: frametime + FPS graphs with min/max/avg stats

The mirror renders into the host window, so the host's render frame timing is
the mirror's performance. FrameStats now retains a ~2 s ring of frame-time
samples; the Video mirror panel plots them as a frametime graph (0-33 ms
scale) and an FPS graph (0-144 scale), each with an avg overlay, and prints
avg/min/max for both frametime and FPS underneath.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/ui/app_chrome.hpp

@@ -4,6 +4,8 @@
 // stay focused on their own pipeline while presenting a consistent shell.
 #pragma once
 
+#include <algorithm>
+
 namespace coop
 {
 
@@ -24,6 +26,9 @@ struct UiState
 class FrameStats
 {
 public:
+	// Number of frame samples kept for the graphs (~2 s at 120 FPS).
+	static constexpr int kHistory = 240;
+
 	void tick(float dt_ms)
 	{
 		if (dt_ms < cur_min_)
@@ -46,8 +51,16 @@ public:
 			accum_ms_ = 0.0f;
 			frames_ = 0;
 		}
+
+		history_[hist_pos_] = dt_ms;
+		hist_pos_ = (hist_pos_ + 1) % kHistory;
+		if (hist_count_ < kHistory)
+		{
+			++hist_count_;
+		}
 	}
 
+	// --- 1 s windowed aggregates (stable readout for the menu bar) ---------
 	[[nodiscard]] float avg_ms() const
 	{
 		return avg_ms_;
@@ -65,6 +78,45 @@ public:
 		return avg_ms_ > 0.0f ? 1000.0f / avg_ms_ : 0.0f;
 	}
 
+	// --- Sample history (for graphs) ---------------------------------------
+	[[nodiscard]] int history_size() const
+	{
+		return hist_count_;
+	}
+
+	// Copy the frame-time samples (ms) into `out` oldest-to-newest; `out` must
+	// hold at least kHistory floats. Returns the number written.
+	int copy_frame_ms(float* out) const
+	{
+		const int start = (hist_pos_ - hist_count_ + kHistory * 2) % kHistory;
+		for (int i = 0; i < hist_count_; ++i)
+		{
+			out[i] = history_[(start + i) % kHistory];
+		}
+		return hist_count_;
+	}
+
+	// min / max / mean over the whole retained history (order-independent).
+	void history_stats(float& min_ms, float& max_ms, float& avg_ms) const
+	{
+		if (hist_count_ == 0)
+		{
+			min_ms = max_ms = avg_ms = 0.0f;
+			return;
+		}
+		float mn = 1.0e9f, mx = 0.0f, sum = 0.0f;
+		for (int i = 0; i < hist_count_; ++i)
+		{
+			const float v = history_[i];
+			mn = std::min(mn, v);
+			mx = std::max(mx, v);
+			sum += v;
+		}
+		min_ms = mn;
+		max_ms = mx;
+		avg_ms = sum / static_cast<float>(hist_count_);
+	}
+
 private:
 	float accum_ms_ = 0.0f;
 	int frames_ = 0;
@@ -73,6 +125,10 @@ private:
 	float avg_ms_ = 0.0f;
 	float min_ms_ = 0.0f;
 	float max_ms_ = 0.0f;
+
+	float history_[kHistory] = {};
+	int hist_pos_ = 0;
+	int hist_count_ = 0;
 };
 
 // Draw the main menu bar (app name, View menu of panel toggles + debug switch,