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

studies/202512_GenesysCX/ctm-token-calculator/tokencalc/rollout.py

@@ -0,0 +1,81 @@
+"""
+Implementation rollout & ramp model.
+
+Captures the gap between **when Genesys starts billing** (contract
+start) and **when each region actually goes live**:
+
+- The platform licence commit bills in full from contract start; the
+  vendor's *ramp period* compensates with a first-year credit
+  (typical: 6-month ramp → 50% Y1 discount on the platform commit).
+- AI token usage (per-user and consumption meters) starts only when a
+  site goes live, and bills for the months the site is live in each
+  model year.
+- Benefits likewise accrue only from go-live (the scenario realization
+  curve then models adoption maturity *within* the live period).
+
+A site with ``go_live_month = m`` is live for ``12*year − m`` months of
+the first ``year`` years (clamped to 0..12 per year). So NAM at month 6
+is live 6 months of Y1; EMEA at month 9 → 3 months; AUZ/APAC at month
+12 → 0 months in Y1 and fully live from Y2.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+MONTHS_PER_YEAR = 12
+
+
+@dataclass
+class RolloutPlan:
+    #: ISO date Genesys starts billing the licence commit (informational,
+    #: surfaced in UI/exports; the model works in months-from-start).
+    contract_start: str | None = None
+
+    #: Total build duration, months (informational).
+    build_months: int = 12
+
+    #: Vendor ramp period, months. Documentation for the Y1 credit below.
+    ramp_months: int = 6
+
+    #: First-year credit on the platform licence commit. Typical
+    #: 6-month ramp = 50% discount in year 1; years 2+ bill in full.
+    first_year_platform_discount: float = 0.5
+
+    #: site_name -> go-live month (months after contract start).
+    #: Sites absent from the map are treated as live from day 0.
+    go_live_month: dict[str, int] = field(default_factory=dict)
+
+    def __post_init__(self) -> None:
+        if not 0.0 <= self.first_year_platform_discount <= 1.0:
+            raise ValueError("first_year_platform_discount must be within 0-1")
+        if self.ramp_months < 0 or self.build_months < 0:
+            raise ValueError("ramp_months/build_months must be >= 0")
+        for site, m in self.go_live_month.items():
+            if m < 0:
+                raise ValueError(f"{site}: go_live_month must be >= 0")
+
+    # ── Availability ────────────────────────────────────────────────
+
+    def live_months_in_year(self, site_name: str, year: int) -> int:
+        """Months ``site_name`` is live during model year ``year`` (1-based)."""
+        go_live = self.go_live_month.get(site_name, 0)
+        live_by_year_end = max(0, MONTHS_PER_YEAR * year - go_live)
+        live_by_prev_year_end = max(0, MONTHS_PER_YEAR * (year - 1) - go_live)
+        return min(MONTHS_PER_YEAR, live_by_year_end - live_by_prev_year_end)
+
+    def fraction_live(self, site_name: str, year: int) -> float:
+        return self.live_months_in_year(site_name, year) / MONTHS_PER_YEAR
+
+    # ── Billing ─────────────────────────────────────────────────────
+
+    def platform_factor(self, year: int) -> float:
+        """Fraction of the full platform commit billed in ``year``."""
+        return 1.0 - self.first_year_platform_discount if year == 1 else 1.0
+
+
+#: Behaviour identical to the pre-rollout model: everything live from
+#: day 0, no ramp credit.
+NO_ROLLOUT = RolloutPlan(
+    build_months=0, ramp_months=0, first_year_platform_discount=0.0
+)