… to _architecture and register legacy module shim

The bundled multitask_model.pt was serialised when MultitaskDeepLCModel and
BatchedHeads lived in a top-level module called `multitask_model`.  That
module no longer exists, so torch.load raised ModuleNotFoundError for any
user who tried to load the default model.

Fix:
- Add BatchedHeads and MultitaskDeepLCModel to deeplc/_architecture.py,
  where the rest of the model architecture already lives.
- Add _patch_legacy_multitask_module() to deeplc/_model_ops.py, which
  registers a sys.modules shim mapping the old import path to the new
  classes before torch.load is called.  The shim is a no-op if the
  module is already registered.
- Add test_load_multitask_model_without_prior_shim to tests/test_model_ops.py
  to prevent regression.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Fix multitask_model.pt loading (ModuleNotFoundError on import)