fix(cv): add per-direction crossing cooldown to suppress track-churn double-counts

When a blob briefly drops below CV_MIN_BLOB_PX, its track is killed and respawns,
causing the same person to generate multiple counts per visit (~50/min observed
in field). Add a per-direction cooldown (default 5 frames ≈ 0.8s @ 5 fps) that
drops subsequent entries (or exits) within the window of the last counted one.
Entry and exit cooldowns are tracked independently.

Fixed at compile time for now; exposing as a server-push tunable is deferred
until the server-push-config branch lands. See docs/server-prompt-crossing-
cooldown.md for the server-side coordination notes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

firmware/lib/cv/cv.h

@@ -12,6 +12,12 @@ static const int CV_MIN_BLOB_PX = 64;
 static const float CV_MAX_MOVE = 15.0f;
 static const int CV_MAX_MISSED = 10;
 
+// Per-direction crossing cooldown. Any same-direction crossing whose frame gap
+// is strictly less than this value is dropped. At 5 fps, a value of 5 → ≈0.8s
+// suppression window. Purpose: mask track churn (blob briefly drops below
+// min_blob_px, track dies & respawns, re-crosses).
+static const uint32_t CV_CROSSING_COOLDOWN_FRAMES = 5;
+
 struct CVTrack {
     int   id;
     float x, y;
@@ -28,6 +34,8 @@ struct CVState {
     std::vector<CVTrack> tracks;
     int entries;
     int exits;
+    uint32_t last_entry_frame; // 0 = never; frame_index of last counted entry
+    uint32_t last_exit_frame;  // 0 = never; frame_index of last counted exit
 };
 
 struct CVResult {