fix(anomaly): exclude non-directional kinds from grading and backtest

Review follow-up (HIGH): the three detectors fed the same evaluator/backtest, but
SuspensionFreeze is non-directional (favourite unchanged) — grading it as "favourite
won" polluted the hit-rate with the base favourite-win rate, and its high frozen-ness
score always cleared the backtest threshold.

- Add AnomalyKind.IsDirectional() (flip + steam = true, freeze = false).
- AnomalyOutcomeEvaluator returns Unresolved for non-directional kinds (favourites
  still surfaced for display) so they don't distort calibration.
- RunBacktestUseCase skips non-directional anomalies when building candidates.
- Tests for the classification, the evaluator path, and the backtest skip.

src/Marathon.Domain/Enums/AnomalyKindExtensions.cs

@@ -0,0 +1,24 @@
+namespace Marathon.Domain.Enums;
+
+/// <summary>Semantic classification of anomaly kinds.</summary>
+public static class AnomalyKindExtensions
+{
+    /// <summary>
+    /// Whether the kind makes a <i>directional</i> prediction — a specific side/favourite
+    /// expected to win — that can be graded against the result and bet on in a backtest.
+    /// </summary>
+    /// <remarks>
+    /// <see cref="AnomalyKind.SuspensionFlip"/> and <see cref="AnomalyKind.SteamMove"/> are
+    /// directional (they point at a favourite). <see cref="AnomalyKind.SuspensionFreeze"/> is
+    /// informational — the line did NOT move — so "predicting" the unchanged favourite would
+    /// merely measure the base favourite-win rate; it is excluded from outcome grading and
+    /// from backtest staking so it does not distort detector calibration.
+    /// </remarks>
+    public static bool IsDirectional(this AnomalyKind kind) => kind switch
+    {
+        AnomalyKind.SuspensionFlip => true,
+        AnomalyKind.SteamMove => true,
+        AnomalyKind.SuspensionFreeze => false,
+        _ => false,
+    };
+}