refactor: restructure repo into core/app modules with per-study folders

Reorganize Palladium codebase into a modular architecture with `core/`
shared logic and `app/` Streamlit UI, separating per-study assets into
`studies/YYYYMM_<Vendor>/` folders containing notebooks, seed data, and
configuration. Update README to reflect new structure, add `.gitignore`
entries for `.env` and study exports, and refresh component documentation.

core/notebook_helpers/__init__.py

@@ -0,0 +1,5 @@
+"""Notebook helpers — pandas tables, plotly charts, IPython display."""
+
+from core.notebook_helpers import charts, display, tables
+
+__all__ = ["charts", "display", "tables"]