fix frame timestamps to use a comparable epoch

steady_clock's epoch is unspecified and typically counts from process start or
boot, so the published timestamp_ns could not be compared against another
node's timestamps, wall clock or a recording — and it recorded when the node
processed the frame rather than when the camera captured it.

Publish two keys on every output instead: device_timestamp_ns from
sl::Camera::getTimestamp(TIME_REFERENCE::IMAGE), and host_timestamp_ns from
system_clock. Both are Unix-epoch nanoseconds, set as dora timestamp
parameters. The SDK clock source is pinned to SYSTEM_CLOCK at construction,
since it is process-wide and decides the epoch of the device value.

Both are kept because the wall clock can step when NTP synchronises after boot
on a Jetson, while the device timestamp is what two nodes reading the same
frame can agree on.

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

src/zed/camera.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <sl/Camera.hpp>
+#include <cstdint>
 #include <string>
 #include <vector>
 
@@ -35,6 +36,7 @@ public:
     void warmup();
     bool grab();
     Calibration calibration() const;
+    int64_t device_timestamp_ns();
     sl::Mat& image();
     sl::Mat& point_cloud();
     sl::Camera& handle();