first commit

src/cost_tracker.py

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+from __future__ import annotations
+
+from dataclasses import dataclass, field
+
+
+@dataclass
+class CostTracker:
+    total_units: int = 0
+    events: list[str] = field(default_factory=list)
+
+    def record(self, label: str, units: int) -> None:
+        self.total_units += units
+        self.events.append(f'{label}:{units}')