Resolve Worker-009..015 code-review findings

Worker-009: WorkerFrameWriter serialized twice and WorkerFrameReader
allocated a payload byte[] per frame. The writer now serializes once into a
single prefix+payload buffer; the reader rents the payload buffer from
ArrayPool and honors the logical frame length.

Worker-010: VariantConverter projected a uint+Time value as a full FILETIME,
producing a near-1601 timestamp. The FILETIME projection is now gated on
`value is long`; uint falls through to the integer projection.

Worker-011: replaced the opaque retryAttempts formula in WorkerPipeClient
with MaxRetryAttempts = int.MaxValue, leaving the connect deadline as the
sole bound.

Worker-012: rewrote stale "future PR / polls on a Timer" comments in
AlarmDispatcher, AlarmCommandHandler, MxAccessAlarmEventSink and
MxAccessEventMapper to match the shipped, post-Worker-001 behavior.

Worker-013 (re-triaged): already resolved — StaMessagePumpTests and
MxAccessStaSessionTests cover the pump and poll loop directly.

Worker-014: moved IAlarmCommandHandler into its own file so
AlarmCommandHandler.cs declares one public type.

Worker-015: clarified the MxAccessBaseEventSink.EnqueueEvent overflow-catch
comment explaining the deliberate double RecordFault no-op.

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

src/MxGateway.Worker/Conversion/VariantConverter.cs

@@ -207,7 +207,14 @@ public sealed class VariantConverter
         MxDataType expectedDataType)
     {
         long longValue = System.Convert.ToInt64(value, CultureInfo.InvariantCulture);
-        if (expectedDataType == MxDataType.Time)
+
+        // The MxDataType.Time projection treats the source as a Windows FILETIME
+        // (a 64-bit 100-ns tick count since 1601). Only a genuine 64-bit source
+        // (long) can carry a valid full FILETIME; a uint can only hold the low
+        // 32 bits, which DateTime.FromFileTimeUtc would silently render as a
+        // near-1601 timestamp. For uint sources fall through to the integer
+        // projection rather than producing a bogus timestamp.
+        if (expectedDataType == MxDataType.Time && value is long)
         {
             return new MxValue
             {