feat(15-01): add PermissionConsolidator static helper

- MakeKey builds pipe-delimited case-insensitive key from UserLogin+PermissionLevel+AccessType+GrantedThrough
- Consolidate groups UserAccessEntry list by key, merges into ConsolidatedPermissionEntry rows
- Empty input short-circuits to Array.Empty
- Output ordered by UserLogin then PermissionLevel for deterministic results

SharepointToolbox/Core/Helpers/PermissionConsolidator.cs

@@ -0,0 +1,59 @@
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Core.Helpers;
+
+/// <summary>
+/// Merges a flat list of UserAccessEntry rows into consolidated entries
+/// where rows with identical (UserLogin, PermissionLevel, AccessType, GrantedThrough)
+/// are grouped into a single row with multiple locations.
+/// </summary>
+public static class PermissionConsolidator
+{
+    /// <summary>
+    /// Builds a pipe-delimited, case-insensitive composite key from the four key fields.
+    /// </summary>
+    internal static string MakeKey(UserAccessEntry entry)
+    {
+        return string.Join("|",
+            entry.UserLogin.ToLowerInvariant(),
+            entry.PermissionLevel.ToLowerInvariant(),
+            entry.AccessType.ToString(),
+            entry.GrantedThrough.ToLowerInvariant());
+    }
+
+    /// <summary>
+    /// Groups entries by composite key and returns consolidated rows.
+    /// Each group's first entry provides UserDisplayName, IsHighPrivilege, IsExternalUser.
+    /// All entries in a group contribute a LocationInfo to the Locations list.
+    /// Results are ordered by UserLogin then PermissionLevel.
+    /// </summary>
+    public static IReadOnlyList<ConsolidatedPermissionEntry> Consolidate(
+        IReadOnlyList<UserAccessEntry> entries)
+    {
+        if (entries.Count == 0)
+            return Array.Empty<ConsolidatedPermissionEntry>();
+
+        return entries
+            .GroupBy(e => MakeKey(e))
+            .Select(g =>
+            {
+                var first = g.First();
+                var locations = g.Select(e => new LocationInfo(
+                    e.SiteUrl, e.SiteTitle, e.ObjectTitle, e.ObjectUrl, e.ObjectType
+                )).ToList();
+
+                return new ConsolidatedPermissionEntry(
+                    first.UserDisplayName,
+                    first.UserLogin,
+                    first.PermissionLevel,
+                    first.AccessType,
+                    first.GrantedThrough,
+                    first.IsHighPrivilege,
+                    first.IsExternalUser,
+                    locations);
+            })
+            .OrderBy(c => c.UserLogin)
+            .ThenBy(c => c.PermissionLevel)
+            .ToList();
+    }
+}