Implement per pixel text edge fades

[gaelcado]

Text inside an EdgeFade currently receives one opacity per glyph, sampled from an estimated font_size * 0.6 bounding box. Small horizontal movements can therefore dim or hide an entire character at once.

Pass the existing device-space EdgeFadeParams through monochrome and subpixel sprite buffers and evaluate the ramp in the fragment shader on Metal, WGPU, and DirectX. Preserve the existing squared ramp and uniform element opacity. SVG icons retain their current whole-primitive behavior and do not get faded twice.

Validation:

• Zeron UI compiles against the local renderer changes, including Metal shader compilation.
• Headless Metal GPU regression test passes: checks opacity at multiple pixels within one glyph-sized mask, a half-pixel edge shift, and the disabled fade case.
• WGPU shader validation and CPU/GPU sprite layout checks passed for both grayscale and subpixel paths.
• Windows renderer CI passes at 53869c2. Fixed missing fade defaults in the Direct3D and scene test fixtures; Direct3D WARP, scene-ordering, SVG tests and the backdrop example build all pass.

Companion to zeronsh/zeron#415, which separately smooths the onset of sidebar overflow fades. Controlled static sidebar fixtures were inspected; live resizing remains unverified.

CPU sanity check: local before/after runs showed unchanged idle usage and no obvious large CPU regression during repeated sidebar redraws. Dev-build samples were noisy, so this is a coarse regression check rather than a precise performance claim.

Visual preview

User-supplied preview; exact running build not recorded.

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