Feat: PolicyCompositionMode enum and exception

Author: rian-be

src/Abstractions/Exceptions/PolicyCompositionConflictException.cs

@@ -0,0 +1,32 @@
+namespace ModularityKit.Mutator.Abstractions.Exceptions;
+
+/// <summary>
+/// Exception thrown when composed policies attempt to produce incompatible outputs.
+/// </summary>
+/// <remarks>
+/// Initializes a new instance of the <see cref="PolicyCompositionConflictException"/> class.
+/// </remarks>
+/// <param name="compositionName">Name of the composed policy set.</param>
+/// <param name="conflictKey">Key or field that conflicted.</param>
+/// <param name="policyNames">Policy names that contributed to the conflict.</param>
+public sealed class PolicyCompositionConflictException(
+    string compositionName,
+    string conflictKey,
+    IReadOnlyList<string> policyNames) : MutationException(
+        $"Policy composition '{compositionName}' has a conflict for '{conflictKey}' between policies {string.Join(", ", policyNames)}.")
+{
+    /// <summary>
+    /// Name of the composed policy set that detected the conflict.
+    /// </summary>
+    public string CompositionName { get; } = compositionName;
+
+    /// <summary>
+    /// Key or field that conflicted during composition.
+    /// </summary>
+    public string ConflictKey { get; } = conflictKey;
+
+    /// <summary>
+    /// Policy names that contributed to the conflicting value.
+    /// </summary>
+    public IReadOnlyList<string> PolicyNames { get; } = policyNames;
+}

src/Abstractions/Policies/PolicyCompositionMode.cs

@@ -0,0 +1,24 @@
+namespace ModularityKit.Mutator.Abstractions.Policies;
+
+/// <summary>
+/// Defines how composed policy set combines child policy decisions.
+/// </summary>
+public enum PolicyCompositionMode
+{
+    /// <summary>
+    /// All composed policies are evaluated and their outputs are merged.
+    /// Any blocking decision blocks the composition.
+    /// </summary>
+    AllOf = 0,
+
+    /// <summary>
+    /// All composed policies are evaluated and at least one allowed branch must succeed.
+    /// Outputs from the allowed branches are merged.
+    /// </summary>
+    AnyOf = 1,
+
+    /// <summary>
+    /// Policies are evaluated in priority order and the first decisive policy wins.
+    /// </summary>
+    Priority = 2
+}