[b-re-w]

Closes #308

• Rule (leader-approved, generic): in _coerce, when an OBJECT parameter receives a proxy class whose type matches (_is_a), the argument is Cls.empty(). The binder contains no Compose name.
• Without a registered factory, empty()'s TypeError, which names register_empty, propagates. An unrelated class is refused as before. Overload voting counts only the type match and does not call Kotlin.
• The temporary wrapper is pinned by _EmptyArgument for the duration of the call (same convention as _BoxedArgument).
• SPEC U-7: Cls == Cls.empty(). It equals omitting the argument only where Kotlin's default is that value. Registering Modifier's factory belongs to pythonx-compose (relayed by the coordinator).
• Tests:
  • PythonxAdapterTest.aProxyClassPassedAsAnObjectArgumentIsItsEmptyValue covers (a) the empty value, (b) no factory, (c) an unrelated type, (d) voting. It is red against the old PythonxAdapter.kt.
  • ProxyClassArgumentRenderTest renders Card(modifier=Modifier) the same as Card(modifier=Modifier.empty()).
• Results: full python-multiplatform desktopTest passes with 0 failures. The compose fixture passes 124/124. androidNative compiles.