feat: Migrate gradient primitive to GPU accelerated SVG filters

[sortedcord]

This PR rewrites the rendering engine of the <DitherGradient> component, migrating it away from a CPU-bound Canvas API loop to a lightweight, GPU-accelerated procedural SVG filter pipeline.

We offload the threshold math to the browser's native compositing engine, this eliminates main-thread blocking on large gradient washes and removes browser anti-aliasing blur. It also unlocks support for custom gradient angles and adjustable quantization levels.

Arbitrary Angle Rotation	Quantization Step Control

New Features

• The direction prop now accepts numbers (0–360 degrees) in addition to cardinal strings ("up", "down", etc.), projecting the gradient vector across the container corners.
• Added a levels prop (default: 6) to allow consumers to control the granularity of the gradient color banding (for example, levels={2} produces a strict 1-bit retro dither).

Key Changes

• Constructs an SVG <linearGradient> and feed it through a dynamic <filter> chain.
• The 4×4 Bayer threshold matrix is now encoded into a tiny Base64 PNG in memory and tiled across the SVG via <feTile>
• Utilizes <feComposite> and <feComponentTransfer type="discrete"> for the stipple math. To prevent channel-bleeding artifacts during transparency dissolves, the filter quantizes a grayscale base and accurately remaps it to the target colors and alpha using an <feColorMatrix>.

Testing

• Verified TypeScript definitions and zero compilation errors.
• Confirmed the component correctly bypasses layout shift hydration mismatches during server-side rendering (SSR).

Closes #19

Summary by Comp AI

No blocking issues found.

_{Written for commit 5bd036b. New commits will trigger a re-review. Generated by Comp AI.}

[tripwire-sh]

passed — nothing tripped. good to merge.

[comp-ai-code-review]

✅ Comp AI code review complete — no issues found.

_{Commit 5bd036b · Posted by Comp AI Code Reviews.}

[comp-ai-code-review]

No blocking issues found across the changed files.

_{Commit 5bd036b · Posted by Comp AI Code Reviews.}