Linearly bin fires so weak heat spots stay round

Dropping each detection whole into its nearest pixel snapped spots by up
to half a pixel and left weak lone fires as squares or plus signs once
thresholded. Share each detection's weight between the four nearest pixel
centres instead (linear binning, Wand 1994): spots sit at the true
position and blur round, at the same build cost.

docs/CURRENT-STATE.md

@@ -2184,7 +2184,9 @@ detections are under 20 MW and stay dim red. Glow sprites and fire-card
 accents read that ramp; confidence and brightness do not affect the color.
 
 At far zoom (camera above ~3,000 km) the layer drapes a Snap Map–style
-heatmap instead of individual markers: the shown detections are binned on a
+heatmap instead of individual markers: the shown detections are linearly binned
+(each detection's weight shared between the four nearest pixel centres, so spots
+sit at the true position and stay round) on a
 global 2048 × 1024 grid (≈20 km per pixel), each weighted by its FRP in megawatts (so heat is
 radiated power, and raising the FRP slider does not dim areas where strong fires
 burn), Gaussian-blurred by one pixel so each fire stays a distinct spot about