mbproxy: remediate the 2026-05-16 code-review findings

Fixes every finding from the codereviews/2026-05-16 multi-agent review
(2 Critical, 20 Major, 38 Minor) and adds that review to the repo.

Highlights: dashboard XSS escape; response cache invalidated on the
write request (not just the response); ReloadValidator now runs at
startup so port collisions / duplicate names / malformed Resilience
profiles fail fast; AdminPort 0 genuinely disables the admin endpoint;
PlcListener accept-loop faults propagate to the supervisor's faulted
path; reconciler Restart builds before removing; Resilience pipelines
are restart-only from a frozen snapshot; multiplexer connect-race leak,
watchdog party-list snapshot, backend-response and FC16 framing
validation; frontend reconnect retry and util.js load guard; plus the
log-event/doc drift sweep and test-port hygiene.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/tests/Mbproxy.Tests/Admin/StatusHubTests.cs

@@ -68,6 +68,28 @@ public sealed class StatusHubTests
         tracker.ActivePlcs().ShouldBeEmpty("no viewer may be stranded after the tab closes");
     }
 
+    [Fact]
+    public async Task Reconnect_SameTab_NewConnectionDisconnectsFirst_DoesNotLeakViewer()
+    {
+        // Mirror of the reconnect test above with the disconnect ordering reversed: the
+        // NEW connection's OnDisconnectedAsync arrives before the old one's. SignalR does
+        // not guarantee the order, so the tracker must be correct either way.
+        var tracker = new PlcSubscriptionTracker();
+
+        var first = MakeHub("conn-old", tracker, out _);
+        await first.SubscribePlc("plc-1", "tab-A");
+
+        var second = MakeHub("conn-new", tracker, out _);
+        await second.SubscribePlc("plc-1", "tab-A");
+
+        await second.OnDisconnectedAsync(null); // the new connection drops first
+        tracker.ActivePlcs().ShouldContain("plc-1",
+            "the tab is still open on the original connection");
+
+        await first.OnDisconnectedAsync(null);
+        tracker.ActivePlcs().ShouldBeEmpty("no viewer may be stranded after the tab closes");
+    }
+
     [Fact]
     public async Task TwoTabs_FirstCloseKeepsActive_LastCloseClears()
     {