Fix linting errors in platform module

- Fix typo: _ensure_chipflow_root → ensure_chipflow_root in signatures.py
- Add missing exports to platform/__init__.py __all__

This fixes F821 (undefined name) and F401 (unused import) linting errors.

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

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

Author: robtaylor

chipflow_lib/platform/__init__.py

@@ -40,9 +40,15 @@
     'SimStep',
     'SoftwareStep',
     'BoardStep',
-    # Simulation platform (accessed via chipflow_lib.platforms.sim)
+    # Platforms
     'SimPlatform',
+    'SiliconPlatform',
+    'SiliconPlatformPort',
+    'SoftwarePlatform',
+    # Base classes
+    'StepBase',
     # IO Signatures
+    'IOSignature',
     'OutputIOSignature',
     'InputIOSignature',
     'BidirIOSignature',
@@ -52,9 +58,19 @@
     'UARTSignature',
     'GPIOSignature',
     'QSPIFlashSignature',
+    # IO Configuration
+    'IOModel',
+    'IOModelOptions',
+    'IOTripPoint',
+    'Sky130DriveMode',
     # IO Utilities
     'attach_data',
     'SoftwareDriverSignature',
     'SoftwareBuild',
-    'Sky130DriveMode',
+    # Utilities
+    'setup_amaranth_tools',
+    'top_components',
+    'get_software_builds',
+    # Schemas
+    'IO_ANNOTATION_SCHEMA',
 ]

chipflow_lib/platform/io/signatures.py

@@ -70,7 +70,7 @@ class BinaryData:
     type: Literal["BinaryData"] = "BinaryData"
 
     def __init__(self, *, filename: Path, offset=0):
-        self.build_dir = _ensure_chipflow_root() / 'build' / 'software'
+        self.build_dir = ensure_chipflow_root() / 'build' / 'software'
         if Path(filename).is_absolute():
             self.filename = filename
         else: