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/text/sdf_atlas.rs

@@ -26,10 +26,19 @@
 #![allow(dead_code)]
 
 use std::collections::HashMap;
+use std::sync::{Mutex, OnceLock};
 
 use crate::scene::text::font_face::Face;
 use crate::scene::text::lff::Glyph;
 
+/// Process-wide glyph atlas, shared by the text collector (which bakes glyphs
+/// while building draw data) and the GPU upload (which reads the texels).
+/// Mirrors the existing global font-glyph caches (see `ttf_glyph`).
+pub fn text_atlas() -> &'static Mutex<GlyphAtlas> {
+    static ATLAS: OnceLock<Mutex<GlyphAtlas>> = OnceLock::new();
+    ATLAS.get_or_init(|| Mutex::new(GlyphAtlas::new(1024, 1024)))
+}
+
 // ── Bake configuration ──────────────────────────────────────────────────────
 
 /// Texels per glyph unit. Glyphs live in a 9-unit cap-height space, so a cap