fix(twincat): review fixes for sum-read and diagnostics (#645)

Strict width+kind eligibility for sum-batched scalars (others read singly); latch only on ADS
1793/1794, split 1792/1797/1798 without latching; 32 KiB per-command budget and 500-item ceiling;
short sum response handled per item. Diagnostics: opt-in diagnosticsEnabled (default off), reuse the
probe's ReadState, sample after the host-state transition, JitterMs renamed HeadroomMs, compact
per-device line so many devices fit the 16-entry cap, unavailable-symbol latch cleared on reconnect.
AdminUI: min/max on sum-read max, diagnostics checkbox. Pure planner extracted and unit-tested.

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Drivers/Forms/TwinCATDriverForm.razor

@@ -54,8 +54,16 @@
             <div class="col-md-4 mb-3">
                 <label class="form-label" for="tcSumReadMax">Sum-read max items</label>
                 <InputNumber id="tcSumReadMax" @bind-Value="_form.SumReadMaxItems" @bind-Value:after="EmitAsync"
-                             class="form-control form-control-sm" />
-                <div class="form-text">Symbols per sum-read command (default 200).</div>
+                             min="1" max="500" class="form-control form-control-sm" />
+                <div class="form-text">Symbols per sum-read command (1-500, default 200).</div>
+            </div>
+            <div class="col-md-4 mb-3">
+                <div class="form-check form-switch mt-4">
+                    <InputCheckbox id="tcDiagnostics" @bind-Value="_form.DiagnosticsEnabled" @bind-Value:after="EmitAsync"
+                                   class="form-check-input" />
+                    <label class="form-check-label" for="tcDiagnostics">PLC diagnostics on /hosts</label>
+                </div>
+                <div class="form-text">Up to 4 extra reads per device per probe tick. Default off (not yet live-gated).</div>
             </div>
         </div>
     </div>
@@ -167,6 +175,7 @@
         public int NotificationMaxDelayMs { get; set; } = 0;
         public bool UseSumRead { get; set; } = false;
         public int SumReadMaxItems { get; set; } = 200;
+        public bool DiagnosticsEnabled { get; set; } = false;
 
         // Probe
         public bool ProbeEnabled { get; set; } = true;
@@ -186,6 +195,7 @@
             NotificationMaxDelayMs = o.NotificationMaxDelayMs,
             UseSumRead = o.UseSumRead,
             SumReadMaxItems = o.SumReadMaxItems,
+            DiagnosticsEnabled = o.DiagnosticsEnabled,
             ProbeEnabled = o.Probe.Enabled,
             ProbeIntervalSeconds = (int)o.Probe.Interval.TotalSeconds,
             ProbeTimeoutSeconds = (int)o.Probe.Timeout.TotalSeconds,
@@ -201,6 +211,7 @@
             NotificationMaxDelayMs = NotificationMaxDelayMs,
             UseSumRead = UseSumRead,
             SumReadMaxItems = SumReadMaxItems,
+            DiagnosticsEnabled = DiagnosticsEnabled,
             Probe = new TwinCATProbeOptions
             {
                 Enabled = ProbeEnabled,