Add pylibcudf source annotations for stubgen

[vyasr]

Description

This PR adds source-level Cython annotations that improve the signatures recovered by stubgen-pyx for pylibcudf.

The changes are source-only and are split out from the generated-stubs/pre-commit branch. They annotate public API contracts that are not recoverable from unannotated Cython alone, including stream-like parameters, structural input protocols, concrete and abstract collection contracts, domain enum aliases, constructor/return annotations for Python def/__cinit__ methods, and public parameter names used in generated stubs.

No generated .pyi files or stub-generation hook changes are included here.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

Validation:

• pre-commit run cython-lint --files $(git diff --name-only -- "*.pyx" "*.pxd")
• git diff --check
• build-pylibcudf-python

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