Reduce code duplication in contiguous.rs prepare functions

[shinaoka]

Summary

strided-einsum2/src/contiguous.rs has 6 nearly identical prepare_* functions:

• prepare_input_view, prepare_input_owned
• prepare_output_view, prepare_output_owned
• (plus backend-generic variants)

All follow the same pattern: extract dims/strides → try_fuse_group() twice → check REQUIRES_UNIT_STRIDE → conditionally copy via HPTT.

Proposal

Consolidate into 2 core functions (prepare_input, prepare_output) with ownership kind as a parameter. Estimated ~200 lines reduction.

Risk

Low. The splitting is purely organizational; semantics remain identical.