Docs: reflect net48; convert tools projects to net48

Documentation now describes the .NET Framework 4.8 stack (was .NET 8):
- README build prerequisites + framework-dependent/no-runtime-install note,
  test count 136 -> 241.
- PLAN.md architecture diagram, stack decision (with PolySharp/shims note),
  suite total 136 -> 241.
- deploy/README-DEPLOY.txt: app is net48 framework-dependent (4.8 is in-box on
  Win10/11), and build prerequisites.

Also migrate the three tools (RioJoySmokeTest, RioSerialMonitor, XcfRegionExtract)
to net48 so they keep building against the now-net48 RioJoy.Core (a net8 project
cannot reference a net48 one). Added PolySharp + System.Memory/System.Text.Json
shims and replaced net-core-only APIs (string.Contains/IndexOf with comparison,
Array.Fill, generic Enum.IsDefined, int.TryParse(span)). All three build clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/RioSerialMonitor/Program.cs

@@ -52,7 +52,7 @@ AnalogReport lastLogged = default;
 bool haveAnalog = false;
 TimeSpan lastAnalogLog = TimeSpan.Zero;
 var min = new short[5]; var max = new short[5];
-Array.Fill(min, short.MaxValue); Array.Fill(max, short.MinValue);
+for (int i = 0; i < min.Length; i++) { min[i] = short.MaxValue; max[i] = short.MinValue; }
 
 void Track(AnalogReport r)
 {
@@ -113,7 +113,7 @@ link.AnalogReceived += r =>
 
 link.VersionReceived += v => { version = v; Log($"VERSION reply: firmware {v}"); };
 link.CheckReceived += c => { checks.Add(c); Log($"CHECK   reply: {c}"); };
-link.ControlReceived += b => { controls++; Log($"control byte 0x{b:X2} ({(Enum.IsDefined((RioControl)b) ? (RioControl)b : (object)"?")})"); };
+link.ControlReceived += b => { controls++; Log($"control byte 0x{b:X2} ({(Enum.IsDefined(typeof(RioControl), (RioControl)b) ? (RioControl)b : (object)"?")})"); };
 link.FramingError += () => { framing++; Log("FRAMING ERROR (resync)"); };
 
 using var cts = new CancellationTokenSource();