Rename AppResponseModel to SelectiveSerializationModel

- Consolidate to using public serialization.py module
- Remove duplicate _appresponse.py file
- Rename AppResponseModel to SelectiveSerializationModel for clarity
- Add documentation explaining the purpose (omitting None fields)
- Addresses gatecat review comment on models.py:39

Author: robtaylor

chipflow_lib/_appresponse.py

@@ -7,7 +7,7 @@
         BaseModel, PlainSerializer, WrapValidator
         )
 
-from .._appresponse import AppResponseModel, OmitIfNone
+from ..serialization import SelectiveSerializationModel, OmitIfNone
 
 class Process(Enum):
     """
@@ -36,9 +36,11 @@ def __str__(self):
           ]
 
 
-class VoltageRange(AppResponseModel):
+class VoltageRange(SelectiveSerializationModel):
     """
-    Models a voltage range for a power domain or IO
+    Models a voltage range for a power domain or IO.
+
+    Optional fields (min, max, typical) are omitted from serialization when None.
     """
     min: Annotated[Optional[Voltage], OmitIfNone()] = None
     max: Annotated[Optional[Voltage], OmitIfNone()] = None

chipflow_lib/config/models.py

@@ -6,9 +6,17 @@
 
 @dataclass
 class OmitIfNone:
+    """Marker for fields that should be omitted from serialization when None."""
     pass
 
-class AppResponseModel(BaseModel):
+class SelectiveSerializationModel(BaseModel):
+    """
+    Base model that supports selective field serialization.
+
+    Fields annotated with OmitIfNone() will be excluded from serialized
+    output when their value is None. This provides cleaner JSON output
+    for optional configuration fields.
+    """
     @model_serializer
     def _serialize(self):
         skip_if_none = set()