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>

docs/superpowers/specs/2026-04-13-door-counter-design.md

@@ -92,10 +92,12 @@ Capture → Grayscale → Downscale 96×96 → Frame diff → Threshold → Blob
 | Blob detect | Connected components; blobs < 8×8 px discarded as noise |
 | Centroid track | Nearest-centroid matching frame-to-frame (max 15px), tracks persist up to 10 missed frames |
 | Line crossing | Virtual horizontal line at configurable vertical position (default: 50% of frame height) |
+| Cooldown | Per-direction cooldown between counted crossings (default 5 frames ≈ 0.8s @ 5 fps) — suppresses duplicate counts from track churn |
 
 **Counting logic:**
 - Centroid crosses line top→bottom = **entry**
 - Centroid crosses line bottom→top = **exit**
+- After a counted entry (resp. exit), subsequent entries (resp. exits) within `CV_CROSSING_COOLDOWN_FRAMES` are ignored. Entry and exit cooldowns are tracked independently, so an entry immediately followed by an exit (or vice versa) still counts both.
 
 Counts accumulate as `{entries, exits}` in RAM and reset each hour on report.