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/__init__.py

@@ -0,0 +1,31 @@
+"""Pure-python TEI financial math."""
+
+from core.calculations.npv import (
+    discount_factor,
+    npv,
+    present_value,
+    present_value_series,
+)
+from core.calculations.payback import payback_months, payback_years
+from core.calculations.roi import roi, roi_percentage
+from core.calculations.scenarios import (
+    SCENARIOS,
+    apply_scenario,
+    risk_adjust_benefit,
+    risk_adjust_cost,
+)
+
+__all__ = [
+    "SCENARIOS",
+    "apply_scenario",
+    "discount_factor",
+    "npv",
+    "payback_months",
+    "payback_years",
+    "present_value",
+    "present_value_series",
+    "risk_adjust_benefit",
+    "risk_adjust_cost",
+    "roi",
+    "roi_percentage",
+]