feat(text): SDF glyph-atlas foundation for shader text rendering

Groundwork to render CAD text as per-glyph SDF quads (box-quad snap,
perf, no LOD) instead of stroke-polyline wires. CPU side fully tested;
GPU side compiles but is not yet wired into the render loop.

- scene::text::sdf_atlas — GlyphAtlas: shelf packer + R8 SDF tiles baked
  per (font, char). Unified field so one shader draws both kinds: TrueType
  = signed distance to the filled outline, LFF stroke = pen band. Uses only
  crates already in the tree (ttf-parser/lyon via the existing Glyph).
- scene::text::glyph_quads — layout_glyph_quads: lays a run out as per-glyph
  quads using the exact tessellate_text_run transform, so quads land where
  the strokes would. Per-char advance (TTF shaping is a follow-up).
- shaders/text.wgsl — SDF fragment shader (smoothstep over fwidth), RTE
  vertex path mirroring image.wgsl, per-vertex colour + draw-order bias.
- scene::pipeline::text_gpu — TextVertex + double-single vertex builder,
  R8 atlas texture upload, bind group, and the image-cloned pipeline.

13 unit tests across the three CPU stages, all green. Modules carry
#![allow(dead_code)] until the render-loop integration lands.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/text/mod.rs

@@ -1,5 +1,7 @@
 pub mod lff;
 pub mod font_face;
+pub mod glyph_quads;
+pub mod sdf_atlas;
 pub mod sysfont;
 pub mod ttf_glyph;
 pub mod web_font;