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.NotificationOutbox/Delivery/DeliveryOutcome.cs

@@ -28,16 +28,19 @@ public record DeliveryOutcome(DeliveryResult Result, string? ResolvedTargets, st
 {
     /// <summary>Creates a successful outcome carrying the resolved delivery targets.</summary>
     /// <param name="resolvedTargets">The concrete delivery targets used, for audit.</param>
+    /// <returns>A <see cref="DeliveryOutcome"/> with <see cref="DeliveryResult.Success"/> and the resolved targets set.</returns>
     public static DeliveryOutcome Success(string resolvedTargets) =>
         new(DeliveryResult.Success, resolvedTargets, null);
 
     /// <summary>Creates a transient-failure outcome carrying an error description.</summary>
     /// <param name="error">Human-readable description of the transient failure.</param>
+    /// <returns>A <see cref="DeliveryOutcome"/> with <see cref="DeliveryResult.TransientFailure"/> and the error set.</returns>
     public static DeliveryOutcome Transient(string error) =>
         new(DeliveryResult.TransientFailure, null, error);
 
     /// <summary>Creates a permanent-failure outcome carrying an error description.</summary>
     /// <param name="error">Human-readable description of the permanent failure.</param>
+    /// <returns>A <see cref="DeliveryOutcome"/> with <see cref="DeliveryResult.PermanentFailure"/> and the error set.</returns>
     public static DeliveryOutcome Permanent(string error) =>
         new(DeliveryResult.PermanentFailure, null, error);
 }

src/ZB.MOM.WW.ScadaBridge.NotificationOutbox/ServiceCollectionExtensions.cs

@@ -31,6 +31,7 @@ public static class ServiceCollectionExtensions
     /// the singleton actor.
     /// </summary>
     /// <param name="services">The DI service collection to register notification outbox services into.</param>
+    /// <returns>The same <paramref name="services"/> instance for chaining.</returns>
     public static IServiceCollection AddNotificationOutbox(this IServiceCollection services)
     {
         ArgumentNullException.ThrowIfNull(services);