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>

docs/PLAN.md

@@ -25,7 +25,7 @@ cockpit's outputs on their behalf.
 ## Target architecture
 
 ```
-   ┌─────────────────────── C# / .NET 8 tray app (x64) ───────────────────────┐
+   ┌────────────────── C# / .NET Framework 4.8 tray app (x64) ─────────────────┐
    │  Serial (RIO protocol)   →  Input mapper (profile)   →  Output router      │
    │   COM port, 9600 8N1         72 inputs + keypads          ├─ Keyboard/Mouse: SendInput (P/Invoke)
    │   packet parse + ACK/NAK     decode iRIO bitfield         ├─ Joystick: HID report → DeviceIoControl ↓
@@ -46,7 +46,11 @@ cockpit's outputs on their behalf.
 - **Virtual joystick:** custom **VHF/UMDF HID driver** (full fidelity: 6 axes,
   96 buttons, 1 hat — exactly the legacy vJoy layout). Not ViGEm (can't hold 96
   buttons).
-- **Stack:** C# / **.NET 8** (LTS), x64. Driver is C (WDK), separate toolchain.
+- **Stack:** C# / **.NET Framework 4.8**, x64 — in-box on Windows 10/11, so
+  deployed builds are framework-dependent with no runtime to install. (Modern C#
+  language features that net48 lacks are supplied by the PolySharp source
+  generator + a few NuGet shims; see `src/RioJoy.Core/Compat/`.) Driver is C
+  (WDK), separate toolchain.
 - **Form:** background **tray app** (NotifyIcon), auto-start with Windows.
 - **Targets:** Windows 10/11 **x64 only**. The legacy x86 / WinXP targets are
   dropped.
@@ -183,7 +187,7 @@ Implemented in `src/RioJoy.Core/Calibration` + `Plasma` (105 xUnit tests total):
 
 ### Phase 5 — Tray app + profiles — code-complete ✅
 Core logic in `src/RioJoy.Core/Profiles` + `RioRuntime`; UI/OS in `src/RioJoy.Tray`
-(136 xUnit tests total across the suite):
+(241 xUnit tests total across the suite):
 - `RioProfile` + `AppConfig` model; `ConfigStore` JSON persistence (round-trip
   tested); `RioIniImporter` ports the legacy `RIO.ini` (buttons/inverts/greeting).
 - `AutoSwitchResolver` + `AutoSwitchWatcher`: the three-state decision