Fix ProcessLoader stale PID validation against kernel process table (#102)

This PR adresses the following issue : `ProcessLoader.ActiveApplication` could return invalid results when `_latestPid` pointed to a process that no longer existed in the kernel's process table. The original exception path was commented out and bypassed (by sh0inx?) with `GetValueOrDefault` to prevent UI lockups, but this only resolved the symptoms without fixing the root cause.

This was due to sevral factors :
- `_latestPid` was never reset or validated against the actual process state
- ProcessLoader maintained its own `_processesByPid` dictionary separate from the kernel's `KernelContext.Processes`
- No cleanups happened when processes exited or were terminated
- ProcessLoader state could drift out of sync with the kernel process table

**Solution/Fixes**
- Validate` _latestPid` against the kernel process table before returning `ActiveApplication`
- Check process state (Exited/Exiting) and automatically clear stale references
- Add thread-safe cleanup methods (`ClearProcess`, `ClearAllProcesses`)
- Integrate `ClearAllProcesses` into Switch.Dispose for proper shutdown cleanup
- Add warning logs when stale PID is detected and cleared for debugging

**Code Changes**:
- `ProcessLoader.cs`: Add `_pidLock`, update `ActiveApplication` with validation, add cleanup methods
- `Switch.cs`: Call `Processes.ClearAllProcesses()` in Dispose()

Reviewed-on: https://git.ryujinx.app/projects/Ryubing/pulls/102

src/Ryujinx.HLE/Switch.cs

@@ -183,6 +183,7 @@ namespace Ryujinx.HLE
         {
             if (disposing)
             {
+                Processes.ClearAllProcesses();
                 System.Dispose();
                 AudioDeviceDriver.Dispose();
                 FileSystem.Dispose();