transfer_material: preserve the user's selection and warn on per-face sources

[weijietan09]

Addresses the first three problems from #6 (the proper per-face re-mapping is left for a follow-up).

• Selection is restored. The old per-object cmds.select(obj) loop left only the last target selected; the original source+targets selection is now captured up front and restored in a finally.
• Long names. cmds.ls(selection=True, long=True), matching create_and_assign_material / find_materials, so ambiguous short names under different parents can't select the wrong objects.
• Single assign call. All targets are selected once and assigned with one cmds.hyperShade(assign=...) — faster, and a single undo entry (same approach as assign_selected_material).
• Multi-material warning. When the source has more than one material (per-face assignment), the first material is still transferred but the status message now says so instead of printing an unqualified success.

I have already installed it in Maya and tested it in my scenario — selection restore on multi-object transfers and the multi-material warning on a per-face source.

[ruxinliang-dev]

Thank you for the detailed report, the implementation, and the Maya testing. I really appreciate the time you put into identifying the issue and preparing this contribution.

I’ve pushed a maintainer follow-up commit on top of your original work. Your original commit and authorship have been preserved.

The follow-up changes:

• cancel multi-material transfers before modifying any targets;
• deduplicate material results;
• group the complete operation into one Maya undo chunk;
• restore the original selection and close the undo chunk even if assignment fails;
• clarify the long-DAG-path explanation;
• add automated coverage for successful assignment, multi-material cancellation, and assignment failure.

All three automated tests pass.

The full per-face material remapping will remain in #8.

Thanks again for the contribution.