feat: wire remaining E2E gaps — account imports, subject transforms, JWT auth, service latency

Close all 5 server-side wiring gaps so E2E tests pass without skips:
- System events: bridge user-defined system_account to internal $SYS
- Account imports/exports: config parsing + reverse response import for cross-account request-reply
- Subject transforms: parse mappings config block, apply in ProcessMessage
- JWT auth: parse trusted_keys, resolver MEMORY, resolver_preload in config
- Service latency: timestamp on request, publish ServiceLatencyMsg on response

src/NATS.Server/Auth/Account.cs

@@ -307,7 +307,7 @@ public sealed class Account : IDisposable
         return new ServiceExportInfo(subject, se.ResponseType, approved, isWildcard);
     }
 
-    public void AddServiceExport(string subject, ServiceResponseType responseType, IEnumerable<Account>? approved)
+    public void AddServiceExport(string subject, ServiceResponseType responseType, IEnumerable<Account>? approved, ServiceLatency? latency = null)
     {
         var auth = new ExportAuth
         {
@@ -318,6 +318,7 @@ public sealed class Account : IDisposable
             Auth = auth,
             Account = this,
             ResponseType = responseType,
+            Latency = latency,
         };
     }
 

src/NATS.Server/Auth/AccountConfig.cs

@@ -5,4 +5,40 @@ public sealed class AccountConfig
     public int MaxConnections { get; init; }      // 0 = unlimited
     public int MaxSubscriptions { get; init; }    // 0 = unlimited
     public Permissions? DefaultPermissions { get; init; }
+
+    /// <summary>Service and stream exports from this account.</summary>
+    public List<ExportDefinition>? Exports { get; init; }
+
+    /// <summary>Service and stream imports into this account.</summary>
+    public List<ImportDefinition>? Imports { get; init; }
+}
+
+/// <summary>
+/// Represents an export declaration in config: exports = [{ service: "sub" }] or [{ stream: "sub" }].
+/// Go reference: server/opts.go — parseExportStreamMap / parseExportServiceMap.
+/// </summary>
+public sealed class ExportDefinition
+{
+    public string? Service { get; init; }
+    public string? Stream { get; init; }
+
+    /// <summary>Optional latency tracking subject (e.g. "latency.svc.echo").</summary>
+    public string? LatencySubject { get; init; }
+
+    /// <summary>Latency sampling percentage (1–100, default 100).</summary>
+    public int LatencySampling { get; init; } = 100;
+}
+
+/// <summary>
+/// Represents an import declaration in config:
+/// imports = [{ service: { account: X, subject: "sub" }, to: "local" }].
+/// Go reference: server/opts.go — parseImportStreamMap / parseImportServiceMap.
+/// </summary>
+public sealed class ImportDefinition
+{
+    public string? ServiceAccount { get; init; }
+    public string? ServiceSubject { get; init; }
+    public string? StreamAccount { get; init; }
+    public string? StreamSubject { get; init; }
+    public string? To { get; init; }
 }