Convert: pycistopic `CistopicObject` pickles

[Claptar]

Part of #20. Depends on #21.

Constraints

• A CistopicObject is a Python pickle of pycisTopic.cistopic_class.CistopicObject. Unpickling needs pycistopic installed (and its scipy/pandas stack) and executes arbitrary code from the file.
• Put it behind an optional extra (pyadata-cli[pycistopic]), import lazily with a clear error when missing, require an explicit --from pycistopic, and document the security caveat.
• Alternative to evaluate: run the unpickling in a subprocess via uv run --with pycisTopic so the core install stays clean and heavy deps never land in the main environment.

Mapping

CistopicObject	AnnData
fragment_matrix (scipy sparse, regions × cells)	X as cells × regions CSR
binary_matrix	layers["binary"]
cell_names, cell_data	obs_names, obs
region_names, region_data	var_names, var
selected_model.cell_topic	obsm["X_topic"]
selected_model.topic_region	varm["topic_region"]
selected_model metrics / parameters	uns["pycistopic"]["model"]
projections	obsm["X_<name>"]
project, path_to_fragments	uns["pycistopic"]

Plan

Unpickle → write through the elements.write helpers. This path is necessarily in memory (the pickle is); document that clearly.

Tests

Fixture pickled in a uv run --with pycisTopic subprocess under the integration marker; check shapes, names and topic matrices.