feat(text): render TEXT as SDF quads behind OCS_TEXT_SDF flag

Wires the SDF glyph atlas into the render loop end-to-end for top-level
TEXT entities, gated by the OCS_TEXT_SDF env var so the default path is
unchanged. Verified rendering on a real drawing: glyph quads land exactly
over the existing stroke text (position, size, rotation), confirming the
atlas → shader → pipeline → screen path and the RTE / UV math.

- text::text_run_placement — extracted from to_truck (behaviour-preserved)
  so the stroke and SDF-quad paths compute an identical run placement.
- sdf_atlas::text_atlas — process-wide GlyphAtlas (Mutex/OnceLock), shared
  by the collector (bakes glyphs) and the GPU upload (reads texels).
- Scene::sdf_text_vertices — collects per-glyph TextVertex for the frame's
  TEXT entities via layout_glyph_quads + push_glyph_vertices. Empty unless
  OCS_TEXT_SDF is set.
- ViewportData.text_verts + Pipeline text_pipeline/atlas/vbuf, upload_text
  (rebuilds the R8 atlas texture when it grows), and a text render pass
  drawn over the wires.

Still additive over the stroke text; only top-level TEXT is wired (MTEXT,
attributes, and block-internal text are next), and the collector re-runs
per frame (vertex caching by geometry epoch is a follow-up).

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

src/scene/pipeline/text_gpu.rs

@@ -22,7 +22,7 @@ use crate::scene::text::sdf_atlas::GlyphAtlas;
 // ── Vertex ────────────────────────────────────────────────────────────────
 
 #[repr(C)]
-#[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
+#[derive(Copy, Clone, Debug, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct TextVertex {
     pub pos: [f32; 3],
     pub pos_low: [f32; 3],