feat: CV module — frame diff + threshold (blob tracking TODO)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/lib/cv/cv.h

@@ -0,0 +1,40 @@
+// firmware/lib/cv/cv.h
+#pragma once
+#include <stdint.h>
+#include <vector>
+
+static const int CV_W = 96;
+static const int CV_H = 96;
+static const int CV_PIXELS = CV_W * CV_H;
+
+static const uint8_t CV_DIFF_THRESH = 30;
+static const int CV_MIN_BLOB_PX = 64;
+static const float CV_MAX_MOVE = 15.0f;
+static const int CV_MAX_MISSED = 10;
+
+struct CVTrack {
+    int   id;
+    float x, y;
+    bool  above_line;
+    int   missed;
+};
+
+struct CVState {
+    uint8_t  background[CV_PIXELS];
+    bool     bg_valid;
+    uint32_t last_motion_frame;
+    uint32_t frame_index;
+    int      next_id;
+    std::vector<CVTrack> tracks;
+    int entries;
+    int exits;
+};
+
+struct CVResult {
+    int entries_delta;
+    int exits_delta;
+};
+
+void cv_init(CVState& state);
+CVResult cv_process(CVState& state, const uint8_t* frame, uint8_t line_pct);
+void cv_reset_counts(CVState& state);