Clarify confusing power pin calculation logic

- Fix line 160: change 'height // 2 + height // 2' to just 'height // 2'
- Add clarifying comment about pin numbering for left side
- The original expression was confusing and simplified to wrong value
- Addresses gatecat review comment on standard.py:160

Author: robtaylor

chipflow_lib/packaging/standard.py

@@ -156,8 +156,8 @@ def _power(self) -> List[PowerPins]:
         # Heuristic for sensible number of power pins for given size
         n = (self.width + self.height) // 12
 
-        # Left side
-        p = self.height // 2 + self.height // 2
+        # Left side (pins 1 to height)
+        p = self.height // 2  # Middle of left side
         assert p > 3
         pins.append(PowerPins(p - 2, p - 1))
         pins.append(PowerPins(p, p + 1, name='d'))