* fix(datasets): remove unreachable timestamp branch in add_frame and document caller contract
- Remove dead code: frame.pop("timestamp") branch in add_frame() could never
execute because validate_frame() raises ValueError for any DEFAULT_FEATURES
key (including timestamp) before we reach that line.
- Expand add_frame() docstring: explicitly document that timestamp and
frame_index must NOT be passed by the caller.
- Add explanatory comment in validate_frame(): clarifies why DEFAULT_FEATURES
are excluded from expected_features, preventing future re-introduction of
the dead branch.
The dead branch originated in #1200, which fixed a shape-(1,) mismatch for a
code path that was subsequently made unreachable by a refactor of validate_frame.
* chore(datasets): narrow PR scope
* fix(datasets): move add_frame timestamp cleanup to dataset_writer
