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/calculations/roi.py

@@ -0,0 +1,27 @@
+"""Return on Investment."""
+
+from __future__ import annotations
+
+
+def roi(benefits_pv: float, costs_pv: float) -> float:
+    """
+    Return on Investment as a fraction.
+
+    ``ROI = (Benefits − Costs) / Costs``
+
+    Costs here are expressed as a positive present-value amount (the absolute
+    cost). Returns ``0.0`` when costs are zero (rather than dividing by zero).
+
+    Example::
+
+        >>> round(roi(101_696_791, 22_983_076), 2)   # Amazon Connect: 342%
+        3.42
+    """
+    if costs_pv <= 0:
+        return 0.0
+    return (benefits_pv - costs_pv) / costs_pv
+
+
+def roi_percentage(benefits_pv: float, costs_pv: float) -> float:
+    """ROI as a percentage (e.g. 342.0 for 342%)."""
+    return roi(benefits_pv, costs_pv) * 100.0