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/npv.py

@@ -0,0 +1,78 @@
+"""
+Net Present Value and discounting.
+
+Convention (matches the Forrester TEI methodology used in the Amazon Connect
+study):
+
+* The *Initial* investment (column "Initial" in TEI tables) is **not**
+  discounted — it occurs at time zero.
+* Year-N cash flows are discounted at the end of the year:
+  ``PV = CF_n / (1 + r) ** n``  for ``n = 1, 2, …``.
+
+That matches the PDF note: *"The initial investment column contains costs
+incurred at 'time 0' or at the beginning of Year 1 that are not discounted.
+All other cash flows are discounted using the discount rate at the end of
+the year."*
+"""
+
+from __future__ import annotations
+
+from collections.abc import Iterable
+
+
+def discount_factor(year: int, discount_rate: float) -> float:
+    """Return ``1 / (1 + r) ** year``. Year 0 → 1.0 (no discount)."""
+    if year < 0:
+        raise ValueError("year must be >= 0")
+    return 1.0 / ((1.0 + discount_rate) ** year)
+
+
+def present_value(amount: float, year: int, discount_rate: float) -> float:
+    """Discount ``amount`` from end-of-year ``year`` to present."""
+    return amount * discount_factor(year, discount_rate)
+
+
+def present_value_series(
+    cashflows: Iterable[float],
+    discount_rate: float,
+    start_year: int = 1,
+) -> float:
+    """
+    Sum the present value of a stream of year-end cashflows.
+
+    Args:
+        cashflows: iterable of year-N amounts (Y1, Y2, …).
+        discount_rate: e.g. 0.10 for 10%.
+        start_year: year of the first element. Default 1 (skip year-0).
+    """
+    total = 0.0
+    for offset, cf in enumerate(cashflows):
+        total += present_value(cf, start_year + offset, discount_rate)
+    return total
+
+
+def npv(
+    cashflows: Iterable[float],
+    discount_rate: float,
+    initial: float = 0.0,
+) -> float:
+    """
+    Net Present Value.
+
+    Args:
+        cashflows: year-end cashflows for Year 1, Year 2, …
+        discount_rate: e.g. 0.10
+        initial: undiscounted year-0 cashflow (negative for a cost,
+            positive for a one-off benefit).
+
+    Returns:
+        ``initial + Σ CF_n / (1 + r)^n``  for ``n = 1..len(cashflows)``.
+
+    Example::
+
+        >>> # Amazon Connect total benefits PV ≈ $101.7M
+        >>> benefits = [27_279_019, 40_333_658, 57_983_794]
+        >>> round(npv(benefits, 0.10) / 1_000_000, 1)
+        101.7
+    """
+    return initial + present_value_series(cashflows, discount_rate, start_year=1)