refactor: use [LibraryImport] for SHEmptyRecycleBin

[laurentiu021]

Summary

Converts the SHEmptyRecycleBin P/Invoke from the runtime-marshalled [DllImport]
to the source-generated [LibraryImport]. Behavior is identical — the generator
emits the same UTF-16, W-suffixed entry-point call, just with the marshalling stub
produced at build time instead of by the runtime.

Change

Services/TuneUpService.cs:

• TuneUpService and its nested NativeMethods are now partial (required so the
  generator can emit the implementation).
• [DllImport("shell32.dll", CharSet = CharSet.Unicode)] static extern →
  [LibraryImport("shell32.dll", StringMarshalling = StringMarshalling.Utf16, EntryPoint = "SHEmptyRecycleBinW")] static partial.

Why identical

• CharSet.Unicode ⇒ StringMarshalling.Utf16 — same UTF-16 string marshalling.
• EntryPoint = "SHEmptyRecycleBinW" names the exact W-suffixed export that
  CharSet.Unicode was already selecting implicitly.
• Signature is otherwise blittable (IntPtr, uint, int HRESULT return) — no
  marshalling behavior changes. Neither the old nor new declaration sets
  SetLastError, so error handling (the hr >= 0 || 0x80070012 check) is unchanged.

Not converted (deliberately)

The other two [DllImport]s — SHGetFileInfo (ref SHFILEINFO, ByValTStr inline
buffers) and SHFileOperation (ref SHFILEOPSTRUCT, LPWStr fields) — pass
non-blittable structs the source generator cannot marshal without a custom
[NativeMarshalling] rewrite, which would not be byte-identical. They stay
[DllImport].

Verification

• Build: main + Tests + IntegrationTests all 0 warnings / 0 errors (the generator
  accepted the signature).

Behavior identical (Protocol Q). refactor: → no release.