Adds `vnasset upscale` CLI command, `VnAssetsSession.upscale()` / `.upscales()`
session methods, and a standalone `vnassets.upscale` module following the
existing remove-bg pattern.
Uses Real-ESRGAN RRDBNet with RealESRGAN_x4plus_anime_6B (~17 MB,
auto-downloaded). A 256x256 warmup tile at load time eliminates ~80s of
first-run CUDA JIT compilation on ROCm. Steady-state: ~1.8s per 1024->2048
upscale on Strix Halo. The upsampler is lazy-loaded on first call and coexists
with SDXL/Qwen in the same session.
Works around a basicsr/torchvision API incompatibility (rgb_to_grayscale moved
in torchvision 0.20+) with a 3-line module shim in upscale.py.
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