Chore: add Track D mini-library package skeleton

src/pystatsv1/trackd/__init__.py

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+"""Track D (Business): accounting-oriented statistics, forecasting, and data analysis.
+
+This subpackage will gradually grow from thin chapter-runner scripts into a reusable
+mini-library: data loading, schema checks, feature engineering helpers, forecasting
+baselines, and report builders.
+"""
+
+from __future__ import annotations
+
+from ._errors import TrackDDataError, TrackDSchemaError  # noqa: F401
+from ._types import DataFrame, DataFrames, PathLike  # noqa: F401
+
+__all__ = [
+    "DataFrame",
+    "DataFrames",
+    "PathLike",
+    "TrackDDataError",
+    "TrackDSchemaError",
+]

src/pystatsv1/trackd/_errors.py

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+"""Exceptions for Track D helpers."""
+
+from __future__ import annotations
+
+
+class TrackDError(Exception):
+    """Base exception for Track D helpers."""
+
+
+class TrackDDataError(TrackDError):
+    """Raised when Track D input data is missing or invalid."""
+
+
+class TrackDSchemaError(TrackDError):
+    """Raised when Track D input data fails schema/contract checks."""

src/pystatsv1/trackd/_types.py

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+"""Shared types for Track D helpers.
+
+These are intentionally lightweight. Track D functions should accept common "path
+like" inputs and return pandas DataFrames.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import TypeAlias
+
+import pandas as pd
+
+PathLike: TypeAlias = str | Path
+DataFrame: TypeAlias = pd.DataFrame
+DataFrames: TypeAlias = dict[str, pd.DataFrame]

tests/test_trackd_imports.py

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+from __future__ import annotations
+
+import importlib
+
+
+def test_trackd_imports() -> None:
+    mod = importlib.import_module("pystatsv1.trackd")
+    assert mod is not None