refactor ZED capture into modular camera class with env config

Extract camera lifecycle (open, warmup, grab, close) and
environment-based configuration into src/zed/camera.{h,cc}.
Thin main.cc now only handles the dora event loop.

Closes #2

src/zed/camera.h

@@ -0,0 +1,50 @@
+#pragma once
+
+#include <sl/Camera.hpp>
+#include <string>
+#include <vector>
+
+namespace zed {
+
+struct Config {
+    int resolution = 720;
+    int fps = 15;
+    std::string depth_mode = "NEURAL";
+    int depth_min_mm = 10;
+    int depth_max_mm = 500;
+    bool depth_fill = false;
+    std::string flip = "ON";
+    int warmup_frames = 30;
+    int retry_interval_ms = 5000;
+
+    static Config from_env();
+};
+
+struct Calibration {
+    float fx, fy, cx, cy;
+    std::vector<double> distortion;
+};
+
+class Camera {
+public:
+    explicit Camera(const Config& config);
+    ~Camera();
+
+    void open();
+    void close();
+    void warmup();
+    bool grab();
+    Calibration calibration() const;
+    sl::Mat& image();
+    sl::Mat& point_cloud();
+
+private:
+    Config config_;
+    sl::Camera zed_;
+    sl::InitParameters init_params_;
+    sl::RuntimeParameters runtime_params_;
+    sl::Mat image_mat_;
+    sl::Mat point_cloud_mat_;
+};
+
+} // namespace zed