feat(backtest): historical strategy backtester

Adds an interactive backtester that replays the SuspensionFlip detector over
all flagged anomalies under a chosen score threshold and staking rule
(flat / percent-of-bankroll / Kelly), and reports the headline numbers a
user needs to judge edge: final bankroll, ROI, max drawdown (peak-to-trough),
win/loss streaks, plus per-bet equity curve.

Domain (pure):
- StakeRule enum + BacktestStrategy params (with validation).
- BacktestSimulator: deterministic function taking strategy + chronological
  candidates → BacktestResult. Implements Kelly with post-flip implied prob
  as p (skipping negative-edge bets), peak-to-trough drawdown tracking, and
  win/loss streak rollups. Mirrors AnomalyOutcomeEvaluator on the 2-way Draw
  guard so tennis data inconsistencies are refused rather than miss-counted.
- Skipped counter split into SkippedByThreshold / SkippedByDataQuality /
  SkippedByBankroll so the UI can distinguish "strategy choice" from
  "data-quality" from "bankroll empty".

Application:
- RunBacktestUseCase: loads anomalies + events + results, parses evidence,
  builds candidates, hands event titles into the simulator so the UI does
  zero repository round-trips of its own.

UI:
- Pages/Anomalies/Backtest.razor: hero, strategy form (MudBlazor — conditional
  sub-field per staking rule), 4-card KPI strip (final bankroll / net profit
  / ROI / max drawdown), counters row, inline-SVG equity curve, trade-trace
  table with per-bet outcome pills and link-back to the source anomaly.
- Nav entry under Analysis. RU + EN i18n.

Tests: +20 (16 simulator math — flat / percent compounding / Kelly +/-
edge / quarter-Kelly / bankroll-exceeded / out-of-order chronology / Draw
favourite / multi-window drawdown / event-title pass-through + 4 use-case
join). All 399 tests pass.

Money rounding switched to MidpointRounding.AwayFromZero throughout the
simulator output for accounting convention.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Marathon.Domain/Backtesting/BacktestStrategy.cs

@@ -0,0 +1,72 @@
+namespace Marathon.Domain.Backtesting;
+
+/// <summary>
+/// Parameters fed to <see cref="BacktestSimulator"/>. The strategy is "for every
+/// SuspensionFlip anomaly with score ≥ <see cref="MinScore"/>, stake
+/// according to <see cref="StakeRule"/> on the post-flip favourite at the
+/// post-flip rate, then settle against the actual <c>EventResult</c>."
+/// </summary>
+/// <param name="StartingBankroll">
+/// Initial bankroll for compounding stake rules. Must be positive.
+/// </param>
+/// <param name="MinScore">
+/// Lower bound on <c>Anomaly.Score</c> — only anomalies at or above this
+/// threshold are bet on. Must be in [0, 1].
+/// </param>
+/// <param name="StakeRule">How to size each bet — see the enum docs.</param>
+/// <param name="FlatStake">
+/// Used when <see cref="StakeRule"/> is <see cref="Backtesting.StakeRule.Flat"/>.
+/// Must be positive.
+/// </param>
+/// <param name="PercentOfBankroll">
+/// Used when <see cref="StakeRule"/> is
+/// <see cref="Backtesting.StakeRule.PercentOfBankroll"/>. Expressed as a
+/// fraction in (0, 1]. e.g. 0.02 = 2 % of bankroll.
+/// </param>
+/// <param name="KellyFraction">
+/// Used when <see cref="StakeRule"/> is <see cref="Backtesting.StakeRule.Kelly"/>.
+/// Multiplier on the raw Kelly fraction; in (0, 1]. 0.25 (quarter-Kelly) is
+/// the conservative default.
+/// </param>
+public sealed record BacktestStrategy(
+    decimal StartingBankroll,
+    decimal MinScore,
+    StakeRule StakeRule,
+    decimal FlatStake,
+    decimal PercentOfBankroll,
+    decimal KellyFraction)
+{
+    public decimal StartingBankroll { get; } = StartingBankroll > 0m
+        ? StartingBankroll
+        : throw new ArgumentOutOfRangeException(nameof(StartingBankroll),
+            StartingBankroll, "StartingBankroll must be positive.");
+
+    public decimal MinScore { get; } = MinScore is >= 0m and <= 1m
+        ? MinScore
+        : throw new ArgumentOutOfRangeException(nameof(MinScore),
+            MinScore, "MinScore must be in [0, 1].");
+
+    public decimal FlatStake { get; } = FlatStake > 0m
+        ? FlatStake
+        : throw new ArgumentOutOfRangeException(nameof(FlatStake),
+            FlatStake, "FlatStake must be positive.");
+
+    public decimal PercentOfBankroll { get; } = PercentOfBankroll is > 0m and <= 1m
+        ? PercentOfBankroll
+        : throw new ArgumentOutOfRangeException(nameof(PercentOfBankroll),
+            PercentOfBankroll, "PercentOfBankroll must be in (0, 1].");
+
+    public decimal KellyFraction { get; } = KellyFraction is > 0m and <= 1m
+        ? KellyFraction
+        : throw new ArgumentOutOfRangeException(nameof(KellyFraction),
+            KellyFraction, "KellyFraction must be in (0, 1].");
+
+    /// <summary>Sensible defaults — flat-stake, score ≥ 0.45, ¼-Kelly waiting in the wings.</summary>
+    public static BacktestStrategy Default { get; } = new(
+        StartingBankroll: 1000m,
+        MinScore: 0.45m,
+        StakeRule: StakeRule.Flat,
+        FlatStake: 50m,
+        PercentOfBankroll: 0.02m,
+        KellyFraction: 0.25m);
+}