Phase 4 Step 3: CSS gradients

[thomasnemer]

Parent: #41

Goal

Implement CSS gradient functions so that elements can use gradient backgrounds (linear, radial, conic, and repeating variants).

Prerequisites

• Step 2 (CSS Transforms) — shares the same render pipeline

Implementation

• Parse linear-gradient(), radial-gradient(), conic-gradient() and their repeating-* variants
• Add Gradient variant to CSS Value enum with color stops representation
• Implement fragment shader evaluation: compute color per-pixel from gradient parameters
• Support gradients in background-image and background shorthand properties
• Handle multiple color stops, color hints, and angle/position syntax

Tests

• Parsing tests for all gradient types and repeating variants
• Color stop interpolation tests
• Fragment shader output correctness tests
• Integration tests with background-image and background shorthand

Acceptance Criteria

• All gradient functions parse correctly with full CSS syntax support
• Gradients render correctly in the fragment shader
• Repeating gradients tile properly
• Gradients work as values for background-image and background