Add CalMS21 Tasks 2 and 3 dataset support

[gchindemi]

Summary

• add CalMS21_Task2 and CalMS21_Task3 to betman fetch_dataset
• fetch the official archives with checksum validation and extract only the train and test keypoint JSON files
• reuse the existing CalMS21-to-movement conversion path
• document dataset layout, filtering, resource requirements, and benchmark limitations
• add focused conversion, round-trip, fetch configuration, and CLI tests

Data representation

Task 2 records retain the CalMS21 annotator hierarchy as task2/<annotator>/<split>/<video-id>.

Task 3 records retain the behavior hierarchy as task3/<behavior>/<split>/<video-id>. Every behavior group remains an independent other-versus-target binary problem; the groups are not combined into a seven-class or fully observed multilabel dataset.

No new in-memory annotation hierarchy, training orchestration, or streaming JSON dependency is introduced.

Validation

• .venv/bin/pytest -q tests/test_calms21.py tests/test_io_core.py — 23 passed
• Ruff checks and formatting — passed
• betman fetch_dataset --help — lists both new dataset IDs
• git diff --check — passed

The broader non-integration suite currently reports 159 passed, 17 failed, and 7 deselected. The failures are in unchanged augmentation configuration, dataset, inference, and training-helper areas and are outside this PR scope.

Closes #95

[Copilot]

Pull request overview

This PR adds first-class support for fetching and converting CalMS21 Task 2 and Task 3 datasets into LISBET’s existing Movement record format, extending the existing CalMS21 ingestion path without introducing a new in-memory annotation model.

Changes:

• Register CalMS21_Task2 and CalMS21_Task3 in the betman fetch_dataset CLI and hub.fetch_dataset.
• Download official Caltech archives with checksum validation and extract only the train/test keypoint JSON members, then reuse calms21.load_taskx for conversion.
• Add targeted tests and user-guide documentation covering layout, filtering, and Task 3’s independent binary behavior groups.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_calms21.py	Adds conversion, round-trip filtering, fetch configuration, and CLI parsing tests for Task 2/3.
src/lisbet/io/ext_sources/calms21.py	Adds a TODO note about potential future streaming conversion (no functional change in this diff hunk).
src/lisbet/hub.py	Extends fetch_dataset to download/validate/unzip Task 2/3 and convert via existing CalMS21 loader.
src/lisbet/cli/commands/fetch.py	Adds Task 2/3 dataset IDs to the CLI choices list.
docs/user_guide/fine_tuning.rst	Documents Task 2/3 dataset layout, filtering examples, and resource/benchmark limitations.

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