docs(punctuation): clarify SenseVoice postprocessing - #3711

Summary

• Correct the CTTransformer API docstring: unpunctuated SenseVoiceSmall output can use punc_model; punctuation restoration is not exclusive to Paraformer.
• Add a dependency-free AST documentation contract and a narrowly path-filtered workflow that checks out the exact PR head with read-only permissions.

Related to #3644. This corrects documentation only; it does not fix semantic punctuation boundaries, change model defaults or close that issue.

Verification

• The new documentation test fails against the original main docstring and passes with this correction.
• 16 existing test_punc_model_none.py tests passed on a source snapshot with the exact modified model file, using the existing torch 2.11 cu128 environment with CUDA hidden. The earlier CPU-environment attempts stalled in dependency imports; their timeout logs were retained and their processes stopped.
• AST equality after removing only the changed class docstring confirms no executable model-code change; syntax compilation and git diff --check passed.
• actionlint 1.7.12 passed for the new workflow.
• Independent review found no P1/P2 issues. Both commits are signed and carry DCO trailers.

The new CI is a documentation contract, not model inference or accuracy validation. No package release or model/dictionary change is included.