Fix output ports to not have output enable signals (_oe)

- Removed _oe signal for output ports
- Updated IOBuffer to only set oe for bidirectional ports
- Updated wire method to handle cases where _oe is None
- Updated tests to match the new behavior

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: robtaylor

chipflow_lib/platforms/silicon.py

@@ -89,7 +89,6 @@ def __init__(self,
                 self._oe = Signal(port.width, name=f"{component}_{name}__oe", init=-1)
             else:
                 self._oe = Signal(1, name=f"{component}_{name}__oe", init=-1)
-        # Output ports don't have _oe signals
 
         logger.debug(f"Created SiliconPlatformPort {name}, width={len(port.pins)},dir{self._direction}")
 
@@ -107,21 +106,21 @@ def wire(self, m: Module, interface: PureInterface):
     def i(self):
         if self._i is None:
             raise AttributeError("SiliconPlatformPort with output direction does not have an "
-                               "input signal")
+                                 "input signal")
         return self._i
 
     @property
     def o(self):
         if self._o is None:
             raise AttributeError("SiliconPlatformPort with input direction does not have an "
-                               "output signal")
+                                 "output signal")
         return self._o
 
     @property
     def oe(self):
         if self._oe is None:
             raise AttributeError("SiliconPlatformPort with output or input direction does not have an "
-                               "output enable signal")
+                                 "output enable signal")
         return self._oe
 
     @property