refactor: split ObjectStateRegistry into its own module

Extract ObjectStateRegistry (1434 lines) from object_state.py (3591 lines)
into object_state_registry.py. object_state.py now contains only FieldProxy
and ObjectState (2167 lines).

Circular import avoided: registry uses TYPE_CHECKING import for ObjectState
(string annotations only), object_state.py imports ObjectStateRegistry at
runtime. __init__.py re-exports from both modules.

Author: trissim

src/objectstate/__init__.py

@@ -68,6 +68,7 @@
     'config',
     'live_context_resolver',
     'token_cache',
+    'object_state_registry',
     'object_state',
     'snapshot_model',
     'parametric_axes',
@@ -222,7 +223,8 @@
 
 
 # Object state management
-from objectstate.object_state import ObjectState, ObjectStateRegistry
+from objectstate.object_state_registry import ObjectStateRegistry
+from objectstate.object_state import ObjectState
 
 # Snapshot model for time-travel
 from objectstate.snapshot_model import Snapshot, StateSnapshot, Timeline