feat: add left-hand-side symmetry reduction

[RHSong]

Summary

• Expose permutation-group element enumeration and group order.
• Add public symmetry lookup through Drudge.get_symm.
• Add symmetry reduction and assembly for tensor definitions.
• Add term-level external-index permutation and orbit-representative utilities.
• Validate that declared LHS symmetries preserve external-index ranges and reconstruct the original definition.
• Use disjoint occupied and virtual bounds in particle-hole drudges, allowing full orbital tensors to be sliced correctly during code generation.
• Add tests for group enumeration, symmetry reduction, reconstruction, invalid symmetry declarations, and seed naming.

This PR provides the Drudge-side support needed for symmetry-aware contraction optimization and code generation. It is separate from PR #84 and contains no mixed fermion-boson or electron-phonon Drudge changes.

A companion Gristmill PR will use these interfaces to:

• optimize only a symmetry-unique seed of the output tensor;
• reconstruct the complete output using inexpensive permutation operations;
• generate blockwise einsum code;
• generate BLAS-based Fortran code.