Address fourth round of Gadfly findings on #76

- Split hexCenters' doc: the count/limit contract had run straight on from
  the #75 anti-regression paragraph with no separator, so its opening "It"
  read as referring to the wrong thing.
- Write the stagger as pitch/2 rather than radius. Same value, but the intent
  is "half a pitch" and only incidentally "one radius".
- fitAxis's step<=0 guard is unreachable from its only caller. Kept, and now
  says so: a helper this small shouldn't need its caller read to be shown
  safe, and the failure mode without it is ±Inf into an int conversion.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

internal/service/ops.go

@@ -214,10 +214,13 @@ type localPoint struct{ x, y float64 }
 // Do not "simplify" this back to anchoring at the region's min corner. That is
 // what #75 was: staggered rows start a full pitch in, and the leftover all lands
 // on the far edge, where clumps hang outside a bed that nothing clips them to.
-// It reports the total alongside the points, and works that total out BEFORE
-// building anything: a fill large enough to be refused shouldn't allocate its
-// whole lattice first just to be counted and thrown away. Over `limit` it
-// returns (nil, total) so the caller can refuse with the real number.
+//
+// # Counting before building
+//
+// hexCenters returns the total alongside the points, and works that total out
+// BEFORE building anything: a fill large enough to be refused shouldn't allocate
+// its whole lattice first just to be counted and thrown away. Over `limit` it
+// returns (nil, total), so the caller can still refuse with the real number.
 func hexCenters(r Region, radius, spacing float64, limit int) ([]localPoint, int) {
 	if radius <= 0 {
 		return nil, 0
@@ -259,7 +262,7 @@ func hexCenters(r Region, radius, spacing float64, limit int) ([]localPoint, int
 		// and centering THAT run puts it exactly half a pitch off its neighbours.
 		// A single-column region has nothing to stagger against.
 		if row%2 == 1 && cols > 1 {
-			n, x = staggered, r.MinX+x0+radius
+			n, x = staggered, r.MinX+x0+pitch/2
 		}
 		for i := 0; i < n; i++ {
 			pts = append(pts, localPoint{x + float64(i)*pitch, y})
@@ -276,6 +279,12 @@ func hexCenters(r Region, radius, spacing float64, limit int) ([]localPoint, int
 // A span too small to hold even one position at that inset still gets one, in the
 // middle: filling a bed narrower than a single plop with one plop is a better
 // answer than refusing to plant it.
+//
+// The step<=0 half of that guard is currently unreachable — hexCenters, the only
+// caller, returns early unless radius > 0, which makes both steps it passes
+// positive. It stays because dividing by a non-positive step yields ±Inf and then
+// a garbage int conversion, and a helper this small should not require reading
+// its caller to know it is safe. Deliberate, not an oversight.
 func fitAxis(length, step, inset float64) (n int, start float64) {
 	if step <= 0 || length < 2*inset {
 		return 1, length / 2