feat(kevin): mention-driven backtest mini-engine

Walks mentions chronologically, T+1 entry, time-based exit per
KevinStrategy. Reuses backtester/metrics::compute_metrics for headline
numbers. KevinPriceLoader fronts market_data + Alpaca.

tests/backtester/test_kevin_backtest.py

@@ -0,0 +1,187 @@
+"""Tests for the mention-driven Kevin backtest mini-engine."""
+
+from datetime import datetime, timedelta, timezone
+from decimal import Decimal
+
+import pandas as pd
+import pytest
+
+from backtester.kevin_backtest import (
+    KevinBacktestParams,
+    KevinBacktestRunner,
+)
+from backtester.metrics import BacktestResult
+from shared.strategies.kevin import KevinStrategy, KevinStrategyConfig
+
+
+class _StubPriceLoader:
+    """In-memory bars; behaves like the real KevinPriceLoader."""
+
+    def __init__(self, bars_by_symbol: dict[str, pd.DataFrame]):
+        self.bars = bars_by_symbol
+        self.spy = bars_by_symbol.get("SPY")
+
+    async def daily_bars(self, symbol, start, end):
+        return self.bars.get(symbol, pd.DataFrame())
+
+    async def is_tradable(self, symbol):
+        return symbol in self.bars
+
+    async def benchmark_bars(self, start, end):
+        return self.spy if self.spy is not None else pd.DataFrame()
+
+
+def _mention(symbol, action, conviction, horizon, days_ago):
+    return type(
+        "M",
+        (),
+        {
+            "id": days_ago,
+            "symbol": symbol,
+            "action": type("A", (), {"value": action})(),
+            "conviction": Decimal(conviction),
+            "time_horizon": type("H", (), {"value": horizon})(),
+            "created_at": datetime(2026, 5, 15, 14, 0, tzinfo=timezone.utc)
+            + timedelta(days=days_ago),
+        },
+    )
+
+
+def _bars(symbol, start_date, prices):
+    """Build a daily-bar DataFrame indexed by date."""
+    dates = pd.date_range(start_date, periods=len(prices), freq="B", tz="UTC")
+    return pd.DataFrame(
+        {
+            "open": prices,
+            "high": prices,
+            "low": prices,
+            "close": prices,
+        },
+        index=dates,
+    )
+
+
+@pytest.fixture
+def cfg() -> KevinStrategyConfig:
+    return KevinStrategyConfig(
+        min_conviction=Decimal("0.6"),
+        max_mention_age_hours=48 * 365,  # effectively no age filter for backtest
+        base_position_pct=Decimal("0.04"),
+        min_trade_usd=Decimal("500"),
+        max_trade_usd=Decimal("5000"),
+        max_per_ticker_usd=Decimal("7500"),
+        hold_days_by_horizon={
+            "days": 3,
+            "weeks": 5,
+            "months": 10,
+            "long_term": 15,
+            "unspecified": 5,
+        },
+        avoid_closes_longs=True,
+        avoid_blocks_days=7,
+    )
+
+
+async def test_backtest_emits_winning_trade(cfg):
+    # NVDA: enters at $100 day 0, exits at $110 day 5 = +10%
+    bars = {
+        "NVDA": _bars("NVDA", "2026-05-15", [100, 102, 104, 106, 108, 110, 112]),
+        "SPY": _bars("SPY", "2026-05-15", [500, 501, 502, 503, 504, 505, 506]),
+    }
+    strategy = KevinStrategy(cfg)
+    mentions = [_mention("NVDA", "buy", "0.8", "weeks", 0)]
+
+    runner = KevinBacktestRunner(strategy, _StubPriceLoader(bars))
+    result = await runner.run(
+        mentions,
+        KevinBacktestParams(
+            initial_capital=Decimal("100000"),
+            slippage_pct=Decimal("0.0005"),
+        ),
+    )
+
+    assert isinstance(result, BacktestResult)
+    assert result.trade_count == 1
+    assert result.total_return_pct > 0
+    # exit was triggered by holding period (5 trading days)
+
+
+async def test_backtest_filters_low_conviction(cfg):
+    bars = {
+        "NVDA": _bars("NVDA", "2026-05-15", [100, 105, 110, 115, 120, 125]),
+        "SPY": _bars("SPY", "2026-05-15", [500] * 6),
+    }
+    strategy = KevinStrategy(cfg)
+    mentions = [_mention("NVDA", "buy", "0.5", "weeks", 0)]  # below 0.6 floor
+
+    runner = KevinBacktestRunner(strategy, _StubPriceLoader(bars))
+    result = await runner.run(mentions, KevinBacktestParams())
+    assert result.trade_count == 0
+
+
+async def test_backtest_dedupe_roll_extends_exit(cfg):
+    # Two BUYs on same ticker within hold window; exit should extend
+    bars = {
+        "NVDA": _bars("NVDA", "2026-05-15", [100] * 20),
+        "SPY": _bars("SPY", "2026-05-15", [500] * 20),
+    }
+    strategy = KevinStrategy(cfg)
+    mentions = [
+        _mention("NVDA", "buy", "0.7", "weeks", 0),
+        _mention("NVDA", "buy", "0.7", "weeks", 3),
+    ]
+    runner = KevinBacktestRunner(strategy, _StubPriceLoader(bars))
+    result = await runner.run(
+        mentions,
+        KevinBacktestParams(dedupe_policy="roll"),
+    )
+    # Exit at day 3 + 5 = 8, not day 0 + 5 = 5
+    assert result.trade_count == 1
+    closed = result.trades[0]
+    assert closed["holding_days_actual"] >= 5
+
+
+async def test_backtest_sell_mid_position_closes_early(cfg):
+    bars = {
+        "NVDA": _bars("NVDA", "2026-05-15", [100, 105, 110, 95, 90, 85, 80]),
+        "SPY": _bars("SPY", "2026-05-15", [500] * 7),
+    }
+    strategy = KevinStrategy(cfg)
+    mentions = [
+        _mention("NVDA", "buy", "0.8", "weeks", 0),
+        _mention("NVDA", "sell", "0.9", "days", 2),
+    ]
+    runner = KevinBacktestRunner(strategy, _StubPriceLoader(bars))
+    result = await runner.run(mentions, KevinBacktestParams())
+    assert result.trade_count == 1
+    assert result.trades[0]["holding_days_actual"] <= 5
+
+
+async def test_backtest_handles_missing_bars(cfg):
+    bars = {
+        "SPY": _bars("SPY", "2026-05-15", [500] * 5),
+        # NVDA missing
+    }
+    strategy = KevinStrategy(cfg)
+    mentions = [_mention("NVDA", "buy", "0.8", "weeks", 0)]
+    runner = KevinBacktestRunner(strategy, _StubPriceLoader(bars))
+    result = await runner.run(mentions, KevinBacktestParams())
+    # Mention skipped (no price data); no trade
+    assert result.trade_count == 0
+
+
+async def test_backtest_computes_alpha_vs_spy(cfg):
+    # NVDA +10%, SPY flat -> positive alpha
+    bars = {
+        "NVDA": _bars("NVDA", "2026-05-15", [100, 100, 100, 100, 100, 110, 110]),
+        "SPY": _bars("SPY", "2026-05-15", [500] * 7),
+    }
+    strategy = KevinStrategy(cfg)
+    mentions = [_mention("NVDA", "buy", "0.8", "weeks", 0)]
+    runner = KevinBacktestRunner(strategy, _StubPriceLoader(bars))
+    result = await runner.run(
+        mentions,
+        KevinBacktestParams(initial_capital=Decimal("100000")),
+    )
+    assert result.alpha_vs_spy_pct is not None
+    assert result.alpha_vs_spy_pct > 0