refactor: reorganize repository to src/ layout

Move 6 packages (core, config, data_processing, analysis, visualization, cli)
into src/ to reduce root clutter. Merge tools/data.py into
data_processing/transforms.py. Move docs to docs/.

Path resolution via .pth file (setup_dev.py), pytest pythonpath config,
and sys.path bootstrap in rxconfig.py and CLI entry points.

Clean up pyproject.toml deps (remove stale pins, add snowflake-connector-python).
Fix tomllib import for Python 3.10 compatibility.

All 113 tests pass.

core/__init__.py

@@ -1,17 +0,0 @@
-"""
-Core module for NHS High-Cost Drug Patient Pathway Analysis Tool.
-
-Contains configuration, models, and shared utilities used across the application.
-"""
-
-from core.config import PathConfig, default_paths
-from core.models import AnalysisFilters
-from core.logging_config import setup_logging, get_logger
-
-__all__ = [
-    "PathConfig",
-    "default_paths",
-    "AnalysisFilters",
-    "setup_logging",
-    "get_logger",
-]