Lay out hands/ like Frametop's other components

trackd/ becomes track/, and the calibration helper the Python tools
import moves from the prototype's folder into tools/. The model
development tools (nettest and the scripts that compare it with the
Python models or cut int8 calibration crops) stay in frame-hands with
the prototype they need.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

hands/track/calib.h

@@ -0,0 +1,39 @@
+// Tracking-camera calibration from the headset's factory files (see tracker/calib.py
+// for the conventions): Kannala-Brandt fisheye intrinsics, and each camera's pose in the
+// head frame (OpenVR's: +x right, +y up, -z forward), metres.
+#pragma once
+
+#include "geom.h"
+
+#include <map>
+#include <string>
+#include <vector>
+
+struct Camera {
+    std::string name;
+    int width = 0, height = 0;
+    double fx = 1, fy = 1, cx = 0, cy = 0, k[4] = {};
+    double R[3][3] = {};   // camera axes (columns) in the head frame
+    V3 origin{};           // camera centre in the head frame
+
+    V2 project_cam(V3 p) const;               // camera frame -> pixels
+    V3 unproject(V2 uv) const;                // pixels -> unit ray, camera frame
+    V3 ray(V2 uv) const;                      // pixels -> unit ray, head frame
+    V2 project(V3 head, double *depth) const; // head frame -> pixels; depth along the optical axis
+    double off_axis(V2 uv) const;             // degrees between the pixel's ray and the axis
+};
+
+// Loads /persist/xrservice.json and /persist/device_config.json. Keyed by calibration
+// name: slam_left, slam_right, upper_left, upper_right.
+bool load_calibration(std::map<std::string, Camera> &out, std::string &err);
+
+// The Arcturus color cameras (tracker/calib.py load_color has the conventions), for
+// fh-camd --with-color's images: luma at 1/scale size, recorded as color_video<N>. They're
+// keyed by those recorded names: left_node is passthrough_left, right_node
+// passthrough_right. crop_subtract: image x = sensor x - the calibration's cropRegion.x.
+// tools/check_color.py tells which node is which and which crop reading fits.
+bool load_color_calibration(std::map<std::string, Camera> &out, const std::string &left_node,
+                            const std::string &right_node, bool crop_subtract, int scale, std::string &err);
+
+// The point closest to several rays (weighted), and its rms distance to them.
+V3 triangulate(const V3 *origins, const V3 *dirs, const double *weights, int n, double *rms);