docs(codec): justify synthetic input field names and nullability

Encoders for scalar, aggregate, and window UDFs build IPC input
schemas from `Field::new(format!("arg_{i}"), dt, true)` — synthetic
names, unconditional nullability. Add a comment at each site
explaining that the field wrapper is only a transport for the
`DataType`: the receiver immediately collapses these fields back to
`Vec<DataType>` when reconstructing `Signature::Exact`, which cannot
encode names or nullability. Setting realistic values here would
be discarded on decode.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

crates/core/src/codec.rs

@@ -481,6 +481,13 @@ fn encode_python_scalar_udf(py: Python<'_>, udf: &PythonFunctionScalarUDF) -> Py
             )));
         }
     };
+    // Input fields exist only as a transport for the per-arg
+    // `DataType`. Names default to `arg_{i}` and nullability to
+    // `true` because the underlying `TypeSignature::Exact` cannot
+    // express either — the receiver immediately collapses these
+    // fields back to `Vec<DataType>` when reconstructing the
+    // `Signature`, so any nullability or metadata set here would be
+    // discarded.
     let input_fields: Vec<Field> = input_dtypes
         .into_iter()
         .enumerate()
@@ -667,6 +674,13 @@ fn encode_python_window_udf(py: Python<'_>, udf: &PythonFunctionWindowUDF) -> Py
             )));
         }
     };
+    // Input fields exist only as a transport for the per-arg
+    // `DataType`. Names default to `arg_{i}` and nullability to
+    // `true` because the underlying `TypeSignature::Exact` cannot
+    // express either — the receiver immediately collapses these
+    // fields back to `Vec<DataType>` when reconstructing the
+    // `Signature`, so any nullability or metadata set here would be
+    // discarded.
     let input_fields: Vec<Field> = input_dtypes
         .into_iter()
         .enumerate()
@@ -795,6 +809,13 @@ fn encode_python_agg_udf(py: Python<'_>, udf: &PythonFunctionAggregateUDF) -> Py
             )));
         }
     };
+    // Input fields exist only as a transport for the per-arg
+    // `DataType`. Names default to `arg_{i}` and nullability to
+    // `true` because the underlying `TypeSignature::Exact` cannot
+    // express either — the receiver immediately collapses these
+    // fields back to `Vec<DataType>` when reconstructing the
+    // `Signature`, so any nullability or metadata set here would be
+    // discarded.
     let input_fields: Vec<Field> = input_dtypes
         .into_iter()
         .enumerate()