perf(ablegacy): coalesce adjacent N/B/F/L scalar reads into one PCCC request

Review 07 P7 (rest): adjacent words on one data file were never coalesced
(one round trip per tag) although the array path proves the transport can read
a span. Within a device pass, scalar tags on one N/B/F/L file within 8
elements of each other (block <= 200 data bytes, under the 236-byte typed-read
limit) now ride one multi-element read and are decoded from their offsets with
exactly the CLR types the single-tag path returns (int / float / bool).

Only shapes the single path decodes identically are coalesced: indirect,
sub-element, array, file-number-less (S:/I:/O:) and width-mismatched tags keep
the single path. A block the device refuses (an authored element past the end
of the file, a native PLC-5 non-N file) falls back to per-tag reads for its
members and its shape is remembered; a comms fault fails the members together.

New driver-config key coalesceReads (default true) with an AdminUI switch
(Razor change: AbLegacyDriverForm, for the Wave 4 live check). Adds an
env-gated ab_server parity case (coalesced == per-tag values).

Refs #738
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

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

@@ -28,6 +28,13 @@
                 </InputSelect>
                 <div class="form-text">Which protocol stack talks to the controller. Default omits the setting (libplctag). Native is opt-in: wire-proven only for N-file reads/writes and F8 reads. Takes effect on the next deploy.</div>
             </div>
+            <div class="col-md-6">
+                <div class="form-check form-switch mt-4">
+                    <InputCheckbox @bind-Value="_form.CoalesceReads" @bind-Value:after="EmitAsync" class="form-check-input" id="ablegacyCoalesceReads" />
+                    <label class="form-check-label" for="ablegacyCoalesceReads">Coalesce adjacent reads</label>
+                </div>
+                <div class="form-text">Read nearby scalar tags on one N/B/F/L file as a single PCCC request. A block the PLC refuses falls back to per-tag reads automatically. Default on.</div>
+            </div>
         </div>
     </div>
 </section>
@@ -97,7 +104,7 @@
     /// <summary>JSON paths this form edits; everything else in the stored config is preserved.</summary>
     internal static readonly string[] OwnedPaths =
     [
-        "transport", "probeTimeoutSeconds",
+        "transport", "probeTimeoutSeconds", "coalesceReads",
         "probe.enabled", "probe.probeAddress",
     ];
 
@@ -163,6 +170,7 @@
         // PLC-5 / LogixPccc exclusion still applies). Never re-emit a concrete default here — that would pin
         // every edited driver to it.
         public AbLegacyTransport? Transport { get; set; }
+        public bool CoalesceReads { get; set; } = true;
 
         // Probe
         public bool ProbeEnabled { get; set; } = true;
@@ -200,6 +208,7 @@
             _loadedProbeTimeout = o.Probe.Timeout,
             TimeoutSeconds = (int)o.Timeout.TotalSeconds,
             Transport = o.Transport,
+            CoalesceReads = o.CoalesceReads,
             ProbeEnabled = o.Probe.Enabled,
             ProbeIntervalSeconds = (int)o.Probe.Interval.TotalSeconds,
             ProbeTimeoutSeconds = (int)o.Probe.Timeout.TotalSeconds,
@@ -220,6 +229,7 @@
             },
             Timeout = DriverConfigMs.Save(TimeoutSeconds, _loadedTimeout),
             Transport = Transport,
+            CoalesceReads = CoalesceReads,
             ProbeTimeoutSeconds = AdminProbeTimeoutSeconds,
         };
     }