Wiresheet: keep the half-and-half knob's white centre centred

Pushed into the wire-coloured half, the centre filled almost all of it: that
half is only KNOB_RADIUS wide and the centre is KNOB_CORE_RADIUS, so at the
real 8px knob size the wire colour was reduced to a sliver and the marker read
as plain grey. Centred, both halves keep enough colour to tell apart.

Found on a slot carrying two links on its own sheet plus one leaving it, which
is the only arrangement that produces this knob and did not occur in testSax
until now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

wiresheet.py

@@ -575,13 +575,13 @@ def draw_knob(x, y, on_sheet, off_sheet, extra_tags=()):
                            outline="white", width=1, tags=tags)
 
     if on_sheet:
-        # A small white centre is what marks the far end as being on this sheet. It
-        # keeps its colour through selection, so the middle never fills in -- and on a
-        # half-and-half knob it sits in the wire-coloured half it belongs to.
-        cx = x - KNOB_RADIUS / 2 if off_sheet else x
+        # A small white centre is what marks the far end as being on this sheet. It keeps
+        # its colour through selection, so the middle never fills in. It stays centred
+        # even on a half-and-half knob: pushed into the wire-coloured half it would fill
+        # that half almost completely and leave no colour to read.
         canvas.create_oval(
-            cx - KNOB_CORE_RADIUS, y - KNOB_CORE_RADIUS,
-            cx + KNOB_CORE_RADIUS, y + KNOB_CORE_RADIUS,
+            x - KNOB_CORE_RADIUS, y - KNOB_CORE_RADIUS,
+            x + KNOB_CORE_RADIUS, y + KNOB_CORE_RADIUS,
             fill="white", outline="", tags=tags + ("link_knob_core",)
         )