Multi-reviewer pass on the autocast wrapper / norm-upcast path:

- Instance-level forward (#2): an instance attribute `model.forward`
  (Unsloth runtime forward patching) shadows class-method overrides, so
  mutating __class__ silently bypassed the wrapper -> fp32 norm met a bf16
  linear with no autocast and crashed. Now wrap the instance attribute when
  present; otherwise subclass as before.
- Wrapper gating (unslothai#5, unslothai#7): install the wrapper iff fp32 norm params actually
  exist (from our upcast, the legacy env upcast, or an external
  _pre_set_compute_dtype policy) -- not on the upcast DECISION. Fixes the
  rollback path leaving external fp32 norms exposed, and stops wrapping models
  with no fp32 norm. Add _unwrap_forward_in_bf16_autocast for re-prepare (unslothai#10).
- config.architectures leak (unslothai#8/unslothai#9): keep the original __name__ on the
  generated subclass (unique __qualname__ for registration) so save_pretrained
  records the base architecture.
- Device detection (unslothai#11): recurse into mapping/list/tuple batches and fall
  back to the model's parameter device instead of defaulting to "cuda".
- Legacy UNSLOTH_UPCAST_LAYERNORM (#1/#3/unslothai#4): route through the shared
  _cast_named_module + union matcher and honour the external-policy deferral.
- Recursive external-ownership guard (unslothai#6): record descendants of tagged
  modules (the external policy casts recursively).
- Fresh-interpreter pickle test (unslothai#12): real subprocess load.

Shared helpers: _find_tensor_device_type, _call_forward_with_bf16_autocast,
_canonical_module_name, _cast_named_module. Unit suite: 25 passed.