Add GPU shader caches (NVIDIA / DirectX) as a junk category

[Vortrix5]

Summary

GPU shader caches build up over time and are always rebuilt automatically, so they're very safe to clear. The main ones:

• %LOCALAPPDATA%\NVIDIA\DXCache and %LOCALAPPDATA%\NVIDIA\GLCache (NVIDIA)
• %LOCALAPPDATA%\D3DSCache (DirectX shader cache)

Add a junk category that covers them.

Where

src/sifty/core/junk.py, alongside the other %LOCALAPPDATA%-based categories (use _local_appdata()).

Acceptance criteria

• New category appears in sifty junk scan
• Covers the NVIDIA and D3D shader cache dirs above (skip any that don't exist on the machine)
• Sandbox test in tests/test_junk.py
• pytest green

Tip

These are pure caches (no user data), so the safety wiring is simple. The /new-junk-category skill walks through it.