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/PlcSubscriptionTrackerTests.cs

@@ -39,6 +39,23 @@ public sealed class PlcSubscriptionTrackerTests
         t.ActivePlcs().ShouldBeEmpty();
     }
 
+    [Fact]
+    public void SameTab_TwoConnections_RemovedNewestFirst_StaysActiveUntilLast()
+    {
+        // Mirror of SameTab_TwoConnections_StaysActiveUntilLastConnectionGone: the
+        // reconnect's NEW connection is the one that drops first (the order is not
+        // guaranteed). The tab must still be alive on the surviving old connection.
+        var t = new PlcSubscriptionTracker();
+        t.SubscribePlc("c-old", "tab", "plc");
+        t.SubscribePlc("c-new", "tab", "plc");
+
+        t.RemoveConnection("c-new");
+        t.ActivePlcs().ShouldContain("plc", "the tab still holds the old connection");
+
+        t.RemoveConnection("c-old");
+        t.ActivePlcs().ShouldBeEmpty();
+    }
+
     [Fact]
     public void DistinctTabs_AreCountedSeparately()
     {