fix(historian-gateway): dispose recorder value-writer channel + clearer OutboxPath warning

Addresses T18 review: GatewayHistorianValueWriter is a DI singleton holding a gRPC
channel — make it IAsyncDisposable so the container closes the channel gracefully at
shutdown. Tighten the blank-OutboxPath warning to state startup will fail.

Claude-Session: https://claude.ai/code/session_012SDSQ3AcaXqPcBtDESBRii

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Historian/ContinuousHistorizationOptions.cs

@@ -75,7 +75,7 @@ public sealed class ContinuousHistorizationOptions
         var warnings = new List<string>();
         if (!Enabled) return warnings;
         if (string.IsNullOrWhiteSpace(OutboxPath))
-            warnings.Add("ContinuousHistorization:OutboxPath is empty while historization is enabled — the durable outbox has no directory to persist to; the recorder cannot be wired.");
+            warnings.Add("ContinuousHistorization:OutboxPath is empty while historization is enabled — the durable outbox has no directory to persist to; host startup will fail when the outbox is constructed. Set an absolute path on durable storage.");
         if (string.Equals(CommitMode, "Periodic", StringComparison.OrdinalIgnoreCase) && CommitIntervalMs <= 0)
             warnings.Add($"ContinuousHistorization:CommitIntervalMs is {CommitIntervalMs} — must be > 0 in Periodic commit mode; the periodic-commit loop cannot run.");
         return warnings;