Fix inconsistent copying of meniscal attributes in BoneMesh.copy()

[Copilot]

The BoneMesh.copy() method was inconsistently handling meniscal attributes: _meniscus_meshes used shallow copy while _meniscal_outcomes and _meniscal_cart_labels were assigned by reference, allowing modifications to the copy to affect the original.

Changes

• Updated _meniscal_outcomes and _meniscal_cart_labels to use .copy() with None checks
• Added empty dict fallback for _meniscus_meshes to match initialization behavior
• All three attributes now follow the same pattern as dict_cartilage_labels

# Before: reference assignment allowed shared state
copy_._meniscal_outcomes = self._meniscal_outcomes

# After: shallow copy prevents shared state
copy_._meniscal_outcomes = (
    self._meniscal_outcomes.copy() if self._meniscal_outcomes else None
)

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