Add Dataset Format

                        IO Module
    ┌───────────────────────────────────────────────────────────┐
    │                                                           │
    │  io/readers/__init__.py                                   │
    │  ├─ DataLoadError         # Custom exception              │
    │  ├─ detect_format()       # Auto-detect from extension    │
    │  ├─ load_records()        # Main dispatcher + normalizer  │
    │  │                                                        │
    │  ├─ _load_jsonl()         # JSONL loader (one obj/line)   │
    │  ├─ _load_json()          # JSON loader (array of objs)   │
    │  ├─ _load_csv()           # CSV loader (tabular)          │
    │  └─ _load_<format>()      # Your new loader               │
    │                                                           │
    │  io/writers/                                              │
    │  └─ graphml.py            # Output conversion (separate)  │
    │                                                           │
    └───────────────────────────────────────────────────────────┘

Data Flow:
  File → detect_format() → _load_<format>() → load_records() → Normalized Records

class DataLoadError(Exception):
    """Raised when data loading fails."""
    def __init__(self, message: str, path: Path, line_number: int | None = None):
        super().__init__(message)
        self.path = path
        self.line_number = line_number

def _load_<format>(path: Path) -> list[dict[str, Any]]:
    """Load records from <Format> file.

    Args:
        path: Path to input file

    Returns:
        List of record dictionaries (unnormalized)

    Raises:
        DataLoadError: If file cannot be parsed
    """

def _load_parquet(path: Path) -> list[dict[str, Any]]:
    """Load records from Parquet file.

    Requires: pip install pandas pyarrow
    """
    try:
        import pandas as pd
    except ImportError:
        raise DataLoadError(
            "Parquet support requires pandas: pip install pandas pyarrow",
            path
        )

    try:
        df = pd.read_parquet(path)
    except Exception as e:
        raise DataLoadError(f"Failed to read Parquet file: {e}", path) from e

    if df.empty:
        raise DataLoadError("Parquet file is empty", path)

    return df.to_dict('records')

def detect_format(path: Path) -> str:
    suffix = path.suffix.lower()
    if suffix == '.jsonl':
        return 'jsonl'
    elif suffix == '.json':
        return 'json'
    elif suffix == '.csv':
        return 'csv'
    elif suffix in ('.parquet', '.pq'):  # Add new extensions
        return 'parquet'
    else:
        raise DataLoadError(
            f"Unknown file format: {suffix}. Supported: .jsonl, .json, .csv, .parquet",
            path
        )

if format_type == 'jsonl':
    records = _load_jsonl(path)
elif format_type == 'json':
    records = _load_json(path)
elif format_type == 'csv':
    records = _load_csv(path)
elif format_type == 'parquet':  # Add new format
    records = _load_parquet(path)

python -c "from kgb.io import load_records; print('OK')"

def test_load_parquet_basic(tmp_path):
    import pandas as pd
    from kgb.io import load_records

    df = pd.DataFrame({
        "doc_id": ["1", "2"],
        "content": ["Text A", "Text B"],
        "meta": ["X", "Y"]
    })

    parquet_file = tmp_path / "test.parquet"
    df.to_parquet(parquet_file)

    records = load_records(
        parquet_file,
        text_field="content",
        id_field="doc_id"
    )

    assert len(records) == 2
    assert records[0]["text"] == "Text A"  # Normalized
    assert records[0]["id"] == "1"


def test_load_parquet_missing_field(tmp_path):
    import pandas as pd
    from kgb.io import load_records, DataLoadError
    import pytest

    df = pd.DataFrame({"wrong_field": ["A", "B"]})
    parquet_file = tmp_path / "bad.parquet"
    df.to_parquet(parquet_file)

    with pytest.raises(DataLoadError, match="Missing text field"):
        load_records(parquet_file)


def test_detect_format_parquet():
    from kgb.io.readers import detect_format
    from pathlib import Path

    assert detect_format(Path("data.parquet")) == "parquet"
    assert detect_format(Path("data.pq")) == "parquet"

doc_id,body,author
1,"Sample text","Alice"

records = load_records(Path("data.csv"), text_field="body", id_field="doc_id")

[{"text": "Sample text", "id": "1", "body": "Sample text", "doc_id": "1", "author": "Alice"}]

# Auto-detect format from extension
kgb extract --input data.parquet --domain legal

# Custom field names
kgb extract --input data.csv --text-field content --id-field doc_id

# Filter specific records
kgb extract --input large.jsonl --domain legal --record-ids REC-001,REC-002