[Feature] SDF font rendering

[vmarcella]

Overview

Add signed distance field (SDF) font rendering for resolution-independent text
that stays crisp at any scale.

Current State

No response

Scope

Goals:

• Generate SDF from font glyphs
• SDF shader for rendering
• Support arbitrary scaling without blur
• Smooth edges at all sizes

Non-Goals:

• Multi-channel SDF (MSDF)
• Precomputed SDF fonts

Proposed API

pub struct SdfGlyphAtlas {
  texture: Texture,
  spread: f32, // SDF spread in pixels
}

impl SdfGlyphAtlas {
  pub fn new(gpu: &Gpu, size: u32, spread: f32) -> Self;
  pub fn get_or_insert(&mut self, font: &Font, char: char) -> &GlyphUV;
}

pub struct SdfTextRenderer {
  // uses SDF-specific shader
}

impl SdfTextRenderer {
  pub fn new(render_context: &mut RenderContext, atlas: &SdfGlyphAtlas) -> Self;
  pub fn draw(
    &mut self,
    layout: &TextLayout,
    position: [f32; 2],
    color: [f32; 4],
    scale: f32,
  ) -> Vec<RenderCommand>;
}

Acceptance Criteria

• SDF generation from rasterized glyphs
• Text scales smoothly without pixelation
• Sharp edges at small and large sizes
• Comparable visual quality to raster at 1x scale
• Example showing text at multiple scales

Affected Crates

lambda-rs

Notes

• SDF spread affects quality vs texture size
• Fragment shader does smoothstep on distance