make point_cloud optional and publish camera_info once per session

ZED_POINT_CLOUD, off by default, gates both retrieveMeasure(XYZRGBA) and the
point_cloud output; when off the measure is not retrieved at all. At 1080p a
full cloud is ~14.7 MB per frame and costs capture time whether or not anything
consumes it, and consumers that need 3D can back-project depth instead.

camera_info now goes out once per camera session rather than on every frame:
fx, fy, cx, cy and the distortion coefficients are fixed for a given camera and
resolution, so repeating them per frame adds no information. It rides the first
frame after the camera opens, and again after a reconnection, so it still
carries that frame's timestamps.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7g2cBJ5e3r15ZbJ9WZn8c

src/zed/camera.h

@@ -14,6 +14,7 @@ struct Config {
     int depth_min_mm = 200;
     int depth_max_mm = 600;
     bool depth_fill = false;
+    bool point_cloud = false;
     int confidence_threshold = 60;
     int texture_confidence_threshold = 100;
     std::string flip = "ON";