feat(reservoir): spread learning to neighbor grid cells for smoother interpolation

Each learn() call now updates a 3×3 neighborhood (center w=1.0, cardinal w=0.3,
diagonal w=0.1). count field changed to float64 to support fractional weights.
Fills grid ~5× faster and eliminates one-sided interpolation at bin boundaries.

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

SNNBot_garage/src/reservoir.nim

@@ -11,7 +11,7 @@ type
   GridCell = object
     sumSin: float
     sumCos: float
-    count:  int
+    count:  float64
 
   LeadGrid* = object
     cells: array[VPERP_BINS * DIST_BINS, GridCell]  # 17 × 8 = 136 cells
@@ -26,7 +26,6 @@ proc cellIndex(vPerp: float, distance: float): int {.inline.} =
 
 proc forward*(grid: var LeadGrid, vPerp, distance: float): float =
   ## Returns circular mean offset in degrees, or -999.0 when no data.
-  let idx = cellIndex(vPerp, distance)
   let vBin = clamp(int(vPerp + 8.5), 0, VPERP_BINS - 1)
   let dBin = clamp(int(distance / DIST_BAND), 0, DIST_BINS - 1)
 
@@ -52,11 +51,24 @@ proc forward*(grid: var LeadGrid, vPerp, distance: float): float =
   result = radToDeg(arctan2(sinSum, cosSum))
 
 proc learn*(grid: var LeadGrid, vPerp, distance, correctOffset: float) =
-  let idx = cellIndex(vPerp, distance)
+  let s = sin(degToRad(correctOffset))
-  grid.cells[idx].sumSin += sin(degToRad(correctOffset))
+  let c = cos(degToRad(correctOffset))
-  grid.cells[idx].sumCos += cos(degToRad(correctOffset))
+  let vBin = clamp(int(vPerp + 8.5), 0, VPERP_BINS - 1)
-  inc grid.cells[idx].count
+  let dBin = clamp(int(distance / DIST_BAND), 0, DIST_BINS - 1)
+  for dv in -1 .. 1:
+    for dd in -1 .. 1:
+      let vb = vBin + dv
+      let db = dBin + dd
+      if vb < 0 or vb >= VPERP_BINS or db < 0 or db >= DIST_BINS: continue
+      let w = if dv == 0 and dd == 0: 1.0
+              elif dv == 0 or dd == 0: 0.3
+              else: 0.1
+      let i = vb * DIST_BINS + db
+      grid.cells[i].sumSin += w * s
+      grid.cells[i].sumCos += w * c
+      grid.cells[i].count  += w
 
 proc totalCount*(grid: LeadGrid): int =
-  for c in grid.cells:
+  var s = 0.0
-    result += c.count
+  for c in grid.cells: s += c.count
+  result = int(s)