feat: add GenesysCX study and fix Streamlit chart key collisions

- Add 202512_GenesysCX TEI study (config, seed data, notebooks, README)
  with NPV $10.8M / ROI 266% including AI-token cost line
- Add explicit `key` parameter to all chart wrappers in app/components
  to prevent StreamlitDuplicateElementId errors when the same figure
  type renders across Summary/Benefits/Costs tabs
- Render benefits bar and cost pie charts on their respective tabs
- Add benefits_vs_costs_by_year chart wrapper

app/components/charts.py

@@ -1,4 +1,9 @@
-"""Streamlit-friendly chart wrappers (delegate to core.notebook_helpers.charts)."""
+"""Streamlit-friendly chart wrappers (delegate to core.notebook_helpers.charts).
+
+Every wrapper takes a ``key`` — the same figure type renders on multiple
+tabs (Summary, Benefits, Costs) within one script run, so Streamlit needs
+explicit element IDs to avoid StreamlitDuplicateElementId errors.
+"""
 
 from __future__ import annotations
 
@@ -7,26 +12,31 @@ import streamlit as st
 from core.notebook_helpers import charts as core_charts
 
 
-def cashflow(yearly_breakdown, *, initial_cost: float = 0.0) -> None:
+def cashflow(yearly_breakdown, *, initial_cost: float = 0.0, key: str = "cashflow") -> None:
     fig = core_charts.cashflow_chart(yearly_breakdown, initial_cost=initial_cost)
-    st.plotly_chart(fig, width="stretch")
+    st.plotly_chart(fig, width="stretch", key=key)
 
 
-def benefits_bar(items) -> None:
+def benefits_bar(items, *, key: str = "benefits_bar") -> None:
     fig = core_charts.benefits_bar(items)
-    st.plotly_chart(fig, width="stretch")
+    st.plotly_chart(fig, width="stretch", key=key)
 
 
-def cost_pie(items) -> None:
+def cost_pie(items, *, key: str = "cost_pie") -> None:
     fig = core_charts.cost_breakdown_pie(items)
-    st.plotly_chart(fig, width="stretch")
+    st.plotly_chart(fig, width="stretch", key=key)
 
 
-def scenario_bars(scenarios) -> None:
+def benefits_vs_costs_by_year(benefit_items, cost_items, *, key: str = "by_year") -> None:
+    fig = core_charts.benefits_vs_costs_by_year(benefit_items, cost_items)
+    st.plotly_chart(fig, width="stretch", key=key)
+
+
+def scenario_bars(scenarios, *, key: str = "scenario_bars") -> None:
     fig = core_charts.scenario_comparison(scenarios)
-    st.plotly_chart(fig, width="stretch")
+    st.plotly_chart(fig, width="stretch", key=key)
 
 
-def waterfall(values) -> None:
+def waterfall(values, *, key: str = "waterfall") -> None:
     fig = core_charts.waterfall(values)
-    st.plotly_chart(fig, width="stretch")
+    st.plotly_chart(fig, width="stretch", key=key)