Phase 8A (2/2): RioJoy.Core multi-targets net48 + net40 (Windows XP flavor)

- TargetFrameworks net48;net40. net48 keeps x64 + ViGEm + System.IO.Ports
  package; net40 adds Microsoft.Bcl.Async + System.ValueTuple and uses the
  in-box SerialPort.
- Compat/TaskCompat bridges Task.Run/Delay/WhenAny/WhenAll (TaskEx on
  net40) and SemaphoreSlim.WaitAsync (net40 blocks briefly - trivial at
  9600 baud).
- IReadOnlyList/IReadOnlyDictionary -> IList/IDictionary throughout
  (net40 predates the IReadOnly* interfaces and the Bcl backport cannot
  make arrays implement them).
- HashCode.Combine replaced with a manual combine (Bcl.HashCode has no
  net40 build); Marshal.SizeOf<T> -> typeof form; ViGEmJoystickSink
  gated #if !NET40. HidFeederJoystickSink stays on both flavors - it
  will drive RioGamepadXP.sys on XP via the same contract.

Both TFMs build; 275 tests green on net48.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/RioJoy.Core/Serial/RioSerialLink.cs

@@ -69,19 +69,19 @@ public sealed class RioSerialLink
         {
             // If any running loop ends (transport closed / error / cancellation),
             // tear the others down too.
-            await Task.WhenAny(loops).ConfigureAwait(false);
+            await Compat.TaskCompat.WhenAny(loops).ConfigureAwait(false);
         }
         finally
         {
             linked.Cancel();
-            await Task.WhenAll(loops.Select(Swallow)).ConfigureAwait(false);
+            await Compat.TaskCompat.WhenAll(loops.Select(Swallow)).ConfigureAwait(false);
         }
     }
 
     /// <summary>Send a pre-built packet (see <see cref="PacketBuilder"/>) to the RIO.</summary>
     public async Task SendAsync(byte[] packet, CancellationToken cancellationToken = default)
     {
-        await _writeLock.WaitAsync(cancellationToken).ConfigureAwait(false);
+        await Compat.TaskCompat.WaitAsync(_writeLock, cancellationToken).ConfigureAwait(false);
         try
         {
             await _transport.WriteAsync(packet, cancellationToken).ConfigureAwait(false);
@@ -191,7 +191,7 @@ public sealed class RioSerialLink
         {
             while (!ct.IsCancellationRequested)
             {
-                await Task.Delay(_options.AnalogPollInterval, ct).ConfigureAwait(false);
+                await Compat.TaskCompat.Delay(_options.AnalogPollInterval, ct).ConfigureAwait(false);
 
                 await RequestAnalogAsync(ct).ConfigureAwait(false);
 

src/RioJoy.Core/Serial/SerialPortTransport.cs

@@ -74,7 +74,7 @@ public sealed class SerialPortTransport : IRioTransport
             // still releases the handle.
         }
 
-        Task close = Task.Run(() =>
+        Task close = Compat.TaskCompat.Run(() =>
         {
             try { _port.Dispose(); }
             catch { /* best-effort release */ }