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>

deploy/README-DEPLOY.txt

@@ -11,8 +11,9 @@ Contents
   postinstall.bat   Entry point (at the root). Elevates, then runs
                     RIOJoy\install-rio.ps1.
   RIOJoy\           The payload folder:
-    app\            The RIOJoy tray application (self-contained .NET 8 — no runtime
-                    install required on the target machine).
+    app\            The RIOJoy tray application (.NET Framework 4.8, framework-
+                    dependent). .NET Framework 4.8 is in-box on Windows 10/11, so
+                    no runtime install is required on the target machine.
     vendor\         The signed ViGEmBus installer (Nefarius, WHQL/attestation-signed).
     install-rio.ps1 The actual install steps (idempotent).
     pre-uninstall.bat  Cleanup entry point. Elevates, then runs uninstall-rio.ps1.
@@ -57,7 +58,8 @@ After install
 
 Building this package (on a dev machine)
 -----------------------------------------
-  Requires the .NET 8 SDK and the signed ViGEmBus installer.
+  Requires the .NET SDK (8.0+) with the .NET Framework 4.8 targeting/developer
+  pack, and the signed ViGEmBus installer.
 
     powershell -ExecutionPolicy Bypass -File deploy\build-package.ps1 `
         -VigemInstaller <path-to-ViGEmBus_x.y.z_x64_x86_arm64.exe>