docs: complete XML doc coverage (returns, summaries, inheritdoc)

Resolve all 622 issues flagged by the enhanced CommentChecker: add missing
<returns> tags (incl. the standard phrasing on non-generic Task methods),
add missing <summary> tags, and replace misused/redundant <inheritdoc/> on
members that override or implement nothing with real documentation.
Documentation-only — no behavior change; solution builds clean.

src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Flattening/FlatteningService.cs

@@ -463,6 +463,7 @@ public class FlatteningService
     /// </summary>
     /// <param name="inheritedJson">The parent template's HiLo trigger JSON, or null.</param>
     /// <param name="derivedJson">The child template's HiLo trigger JSON override, or null.</param>
+    /// <returns>The merged HiLo config JSON, or the derived config on parse failure of either input.</returns>
     public static string? MergeHiLoConfig(string? inheritedJson, string? derivedJson)
     {
         if (string.IsNullOrWhiteSpace(inheritedJson)) return derivedJson;
@@ -526,6 +527,7 @@ public class FlatteningService
     /// </summary>
     /// <param name="inheritedJson">The parent template's HiLo trigger JSON, or null.</param>
     /// <param name="editedJson">The user-edited HiLo trigger JSON, or null.</param>
+    /// <returns>A JSON string containing only the keys that differ from the inherited config, or null if there are no differences.</returns>
     public static string? DiffHiLoConfig(string? inheritedJson, string? editedJson)
     {
         if (string.IsNullOrWhiteSpace(editedJson)) return null;

src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Flattening/RevisionHashService.cs

@@ -34,6 +34,7 @@ public class RevisionHashService
     /// excluding volatile fields like GeneratedAtUtc.
     /// </summary>
     /// <param name="configuration">The flattened configuration to hash.</param>
+    /// <returns>A hex-encoded SHA-256 hash string of the canonical configuration representation.</returns>
     public string ComputeHash(FlattenedConfiguration configuration)
     {
         ArgumentNullException.ThrowIfNull(configuration);