Fix missing io import in silicon.py

- Added import from amaranth.lib for io module
- Fixes F821 Undefined name 'io' errors in the linter

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

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

Author: robtaylor

chipflow_lib/platforms/silicon.py

@@ -7,6 +7,7 @@
 
 from amaranth import Module, Signal, Cat, ClockDomain, ClockSignal, ResetSignal
 
+from amaranth.lib import io
 from amaranth.lib.cdc import FFSynchronizer
 from amaranth.lib.wiring import PureInterface