Initial extraction from monorepo

fire_planner/strategies/base.py

@@ -0,0 +1,37 @@
+"""Withdrawal-strategy abstract base.
+
+All strategies operate in REAL GBP terms — the simulator deflates by
+the cumulative CPI index before calling. Brackets inside the tax
+engines are also assumed to inflate with CPI (simplifying assumption
+that tax thresholds keep pace with inflation — fiscal drag is a
+documented v2 follow-up).
+"""
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class StrategyState:
+    """Inputs to a strategy's per-year decision. Real GBP throughout."""
+    portfolio: float
+    initial_portfolio: float
+    initial_withdrawal: float
+    year_idx: int
+    horizon_years: int
+    last_withdrawal: float
+    expected_real_return: float = 0.04
+
+
+class WithdrawalStrategy(ABC):
+    name: str
+
+    @abstractmethod
+    def propose_withdrawal(self, state: StrategyState) -> float:
+        """Return the proposed withdrawal in real GBP for this year.
+
+        The simulator may clip downward if the portfolio is exhausted —
+        strategies can request more than the portfolio holds.
+        """
+        raise NotImplementedError