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/Proxy/Multiplexing/MultiplexerE2ETests.cs

@@ -342,6 +342,9 @@ public sealed class MultiplexerE2ETests
             ["Mbproxy:Connection:BackendConnectTimeoutMs"] = "3000",
             // Long request timeout so the watchdog doesn't fire during the test's wait window.
             ["Mbproxy:Connection:BackendRequestTimeoutMs"] = "30000",
+            // This test exercises backend disconnect, not keepalive — disable keepalive so
+            // the 30 s request timeout above doesn't trip the heartbeat cross-field rule.
+            ["Mbproxy:Connection:Keepalive:Enabled"] = "false",
             // Aggressive backend retry so the second connect happens fast.
             ["Mbproxy:Resilience:BackendConnect:MaxAttempts"] = "5",
             ["Mbproxy:Resilience:BackendConnect:BackoffMs:0"] = "50",
@@ -458,8 +461,11 @@ public sealed class MultiplexerE2ETests
         var config = MakeBaseConfig(proxyPort);
         config["Mbproxy:AdminPort"] = adminPort.ToString();
         // Short idle window so the heartbeat fires several times within the test budget.
+        // BackendRequestTimeoutMs is lowered below the 700 ms idle window so the
+        // heartbeat cross-field rule (idle > request timeout) holds.
         config["Mbproxy:Connection:Keepalive:Enabled"] = "true";
         config["Mbproxy:Connection:Keepalive:BackendHeartbeatIdleMs"] = "700";
+        config["Mbproxy:Connection:BackendRequestTimeoutMs"] = "500";
 
         var host = BuildBcdHost(config);
         using var startCts = new CancellationTokenSource(TimeSpan.FromSeconds(3));