Add note about use of BaseModel.construct

The previous version used by the direct() methods
would be something like:

        def model_construct(cls, _fields_set: set[str] | None = None, **values: Any) -> Self:
            node = cls.__new__(cls)
            setter = object.__setattr__
            for k, v in values.items():
                setter(node, k, v)
            if _fields_set is None:
                _fields_set = set(values.keys())
            setter(node, "__fields_set__", _fields_set)
            return node

Testing with 100MB graph indicates that construct() might be
fractionally slower than implementing it ourselves as above
but it is not conclusive. The direct() method only accounts
for 14% of the graph load time.

python/lsst/daf/butler/_compat.py

@@ -176,6 +176,9 @@ def model_dump_json(
 
         @classmethod  # type: ignore
         def model_construct(cls, _fields_set: set[str] | None = None, **values: Any) -> Self:
+            # BaseModel.construct() is very close to what we previously
+            # implemented manually in each direct() method but does have one
+            # extra loop in it to fill in defaults and handle aliases.
             return cls.construct(_fields_set=_fields_set, **values)
 
         @classmethod