Software renderer: paint to CG bitmap #36

Phase 3, Issue 6: Software Renderer#

Implement a software renderer in the render crate that paints the layout tree into a CoreGraphics bitmap.

Requirements#

Display list generation:

• Walk the layout tree and generate a flat list of paint commands
• Paint commands: FillRect(x, y, w, h, color), DrawGlyph(x, y, glyph_bitmap, color)
• Z-order: paint in tree order (background first, then text)

Painting:

• Block backgrounds: fill rectangle with background color (white default)
• Text: render each glyph bitmap from the text crate into the CG bitmap
• Text color: black default
• Blend glyph coverage (grayscale anti-aliasing) with text color onto background

API:

• Renderer::new(width: u32, height: u32) -> Renderer
• Renderer::paint(layout_tree: &LayoutTree, font: &Font)
• Renderer::pixels() -> &[u8] — BGRA pixel data ready for CoreGraphics
• Or: render_to_bitmap(layout: &LayoutTree, font: &Font, width: u32, height: u32) -> BitmapContext

Color:

• Simple Color struct with r, g, b, a (u8 each)
• Predefined: Color::BLACK, Color::WHITE

Acceptance criteria#

• Renders a simple page with text visible in the output bitmap
• Block backgrounds are painted
• Text glyphs are composited with anti-aliasing
• Output is BGRA format compatible with platform crate's BitmapContext
• cargo clippy -p we-render -- -D warnings passes
• cargo test -p we-render passes
• No unsafe code in render crate (unsafe only in platform crate for CG calls)
• No external dependencies