Add Python user_data access

[gulldan]

Summary

• Expose Morpheme.user_data() in the Python bindings for both analyzed morphemes and standalone dictionary-entry morphemes.
• Add user_data to Python field subsetting, type stubs, and subsetting docs, including an example for partial loading.
• Also fix stale subsetting docs that referred to splits_a / splits_b; accepted field names are split_a / split_b.
• Cover user-data access from tokenization, Dictionary.lookup(), Dictionary.entries(), lookup_all_entries(), and dictionaries built from user CSVs.

Root cause

The v0.7 Rust core already stores and resolves user_data, but the Python Morpheme API and Python field-subset parser did not expose or load that field.

Validation

• cargo fmt --all
• git diff --check
• cargo test -p sudachi user_data
• cargo test -p sudachipy
• (cd python && python -m pip install -e .)
• python -m unittest python.tests.test_build python.tests.test_morpheme python.tests.test_dictionary python.tests.test_tokenizer
• python -m unittest python.tests.test_dictionary python.tests.test_morpheme
• cargo test --workspace
• python -m unittest discover python/tests

Closes #338