feat(gpu): Level Zero accelerated blur kernel + root compat aliases

[zzd1314]

Two changes, both needed by downstream users of this library:

1. GPU gaussian blur kernel (2d/total)

Complete Level Zero implementation of the blur accelerator (the author mentioned partial support previously, this fills it in):

• build/blur_ocl.cl: OpenCL C two-pass kernel (blurh/blurv), premultiplied RGBA8 input, clamped edges, normalized Gaussian weights (radius = ceil(3*sigma), capped at 64)
• blur_ocl.go: full Level Zero pipeline via gozel (module/context/command list, two kernel launches, event sync, USM shared buffers)
• blur.go: Blur() auto-uses the GPU when a Level Zero device is available and falls back to imaging.Blur on CPU for any error (no Level Zero / unsupported iGPU / runtime failure)
• effects.go: route Context.Blur through the new total.Blur
• Only .cl + compiled _.spv are committed (go:embed needs the spv); the .spt text disassembly could not be regenerated on this machine (no llvm-spirv), reviewers can regenerate with go generate + llvm-spirv

Verified: all existing tests pass; GPU/CPU blur consistency test passes on GPU-capable hardware (skipped gracefully when no Level Zero device, e.g. Haswell iGPU).

2. Root package compat aliases (compat.go)

The API restructure moved Radians/ImageToNRGBA/TakeThemeColorsKMeans into the 2d/unit and 2d/total subpackages, breaking downstream modules that still call the old root-package signatures (e.g. FloatTech/rendercard). This adds root-level aliases with the old signatures (no error return) so downstream code compiles unchanged.