doc: explicitly document that id_col defines the output column name

[Copilot]

id_col in all three calculators acts as both an input column selector and an output schema contract — the returned DataFrame carries that exact column name — but this was never stated in any docstring or API reference, causing silent schema surprises in downstream pipelines.

Changes

• Docstrings (comorbidity(), hfrs(), disability()): expanded id_col description to state explicitly that the value is used verbatim as the output column name; no renaming occurs.
• docs/api.md: same note added to all three function entries; Returns bullets updated from "Patient ID (original column name)" → "Patient ID (column name matches id_col parameter)".
• Test: added test_custom_id_col_name_preserved_in_output to assert that a non-default id_col (e.g. "PATIENT_ID") appears as the first column of the result.

Behaviour (unchanged)

df = pl.DataFrame({"PATIENT_ID": ["P001", "P001"], "ICD10_CODE": ["I21", "I50"]})
result = comorbidity(df, id_col="PATIENT_ID", code_col="ICD10_CODE")
assert result.columns[0] == "PATIENT_ID"  # always was true; now documented

Warning

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