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>

README.md

@@ -27,8 +27,11 @@ Red Planet — talk to the RIO directly and do not use this app.)
 
 ## Building
 
-Requires the **.NET 8 SDK** and Windows. The driver builds separately with the
-**WDK** (see [`driver/README.md`](driver/README.md)).
+Requires the **.NET SDK** (8.0 or newer) plus the **.NET Framework 4.8
+targeting/developer pack**, on Windows. The apps target **.NET Framework 4.8**,
+which is in-box on every Windows 10/11 machine — so deployed builds are
+framework-dependent and need **no runtime install** on the target. The driver
+builds separately with the **WDK** (see [`driver/README.md`](driver/README.md)).
 
 ```sh
 dotnet build RioJoy.sln -c Release
@@ -37,7 +40,7 @@ dotnet test RioJoy.sln
 
 ## Status
 
-Phases 1–5 are implemented and tested (136 unit tests). The `RioGamepad` virtual
+Phases 1–5 are implemented and tested (241 unit tests). The `RioGamepad` virtual
 HID driver is built (KMDF + VHF), **test-signed, installed, and verified**: it
 enumerates in `joy.cpl`, and the C# HID feeder (`DeviceIoControl` →
 `RioGamepad.sys`) drives its axes, buttons, and hat end-to-end (see