fix(localdb): adapter review fixes — deterministic pump teardown, always-hosted initiator, backoff clamp

Claude-Session: https://claude.ai/code/session_01BL2Vu1ESDQ9SCN4gVKkdts

ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/ReplicationOptionsValidator.cs

@@ -9,6 +9,9 @@ namespace ZB.MOM.WW.LocalDb.Replication;
 /// </summary>
 public sealed class ReplicationOptionsValidator : IValidateOptions<ReplicationOptions>
 {
+    // Also what keeps SyncBackgroundService's unchecked backoff doubling overflow-safe.
+    private static readonly TimeSpan ReconnectBackoffCeiling = TimeSpan.FromDays(1);
+
     public ValidateOptionsResult Validate(string? name, ReplicationOptions options)
     {
         var failures = new List<string>();
@@ -31,6 +34,8 @@ public sealed class ReplicationOptionsValidator : IValidateOptions<ReplicationOp
             failures.Add($"LocalDb:Replication:TombstoneRetention must be greater than zero; got {options.TombstoneRetention}.");
         if (options.ReconnectBackoffMax <= TimeSpan.Zero)
             failures.Add($"LocalDb:Replication:ReconnectBackoffMax must be greater than zero; got {options.ReconnectBackoffMax}.");
+        else if (options.ReconnectBackoffMax > ReconnectBackoffCeiling)
+            failures.Add($"LocalDb:Replication:ReconnectBackoffMax must be at most {ReconnectBackoffCeiling}; got {options.ReconnectBackoffMax}.");
         if (options.MaxHlcDriftAhead <= TimeSpan.Zero)
             failures.Add($"LocalDb:Replication:MaxHlcDriftAhead must be greater than zero; got {options.MaxHlcDriftAhead}.");