Force file sources to decode from frame 0; add first-frame diagnostics

OpenCV FFMPEG build buffer (CAP_PROP_BUFFERSIZE=1) on file sources was
landing mid-file, so the first decoded frame could show an ~11s-later
camera clock than the source's true frame 0. Gate BUFFERSIZE to live
sources only and seek file sources to CAP_PROP_POS_FRAMES=0. Log
pos_frames/pos_msec/frame_count/fps once on first successful read.

SPEC: B1, V18, T20. AGENTS: capture setup notes.

src/main.cpp

@@ -41,7 +41,11 @@ static cv::VideoCapture open_capture(const std::string& source) {
         setenv("OPENCV_FFMPEG_CAPTURE_OPTIONS", g_config.rtsp_ffmpeg_options.c_str(), 1);
     }
     cv::VideoCapture cap(source, cv::CAP_FFMPEG);
-    cap.set(cv::CAP_PROP_BUFFERSIZE, 1);
+    if (is_live) {
+        cap.set(cv::CAP_PROP_BUFFERSIZE, 1);
+    } else {
+        cap.set(cv::CAP_PROP_POS_FRAMES, 0);
+    }
     return cap;
 }
 
@@ -291,6 +295,7 @@ int main(int argc, char* argv[]) {
     int reconnect_count = 0;
     bool blank_live_frame = false;
     bool paused = false;
+    bool first_read_logged = false;
 
     auto write_live_jpeg = [](const cv::Mat& out_frame, int quality) {
         try {
@@ -335,6 +340,16 @@ int main(int argc, char* argv[]) {
             continue;
         }
 
+        if (!first_read_logged) {
+            first_read_logged = true;
+            std::cout << "[" << now_str() << "] [INFO] First frame decoded — "
+                      << "pos_frames=" << cap.get(cv::CAP_PROP_POS_FRAMES)
+                      << " pos_msec=" << cap.get(cv::CAP_PROP_POS_MSEC)
+                      << " frame_count=" << cap.get(cv::CAP_PROP_FRAME_COUNT)
+                      << " fps=" << cap.get(cv::CAP_PROP_FPS)
+                      << std::endl;
+        }
+
         auto elapsed_sec = std::chrono::duration<double>(
             std::chrono::steady_clock::now() - session_start).count();
         auto mono = std::chrono::steady_clock::now();