feat(text): make SDF text the default renderer

Flip `sdf_text_enabled()` to default ON: text renders as SDF glyph quads
(crisp at every zoom, no LOD, box-based snap/pick) for everyone. Set
`OCS_TEXT_SDF=0` to fall back to the legacy stroke path — a kill switch kept
in case a drawing or environment needs the old renderer. Off-screen text is
still view-culled per entity, so dropping the text LOD ladder stays bounded.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/text/sdf_atlas.rs

@@ -39,12 +39,13 @@ pub fn text_atlas() -> &'static Mutex<GlyphAtlas> {
     ATLAS.get_or_init(|| Mutex::new(GlyphAtlas::new(1024, 1024)))
 }
 
-/// Whether SDF text rendering is enabled (env `OCS_TEXT_SDF`), read once.
-/// When on, the wire path suppresses text glyph strokes so text draws only as
-/// SDF quads (no double draw); the insertion snap point is kept.
+/// Whether SDF text rendering is enabled, read once. Default ON — text draws
+/// as SDF glyph quads (crisp at every zoom, no LOD; the wire path suppresses
+/// the glyph strokes and keeps the insertion snap). Set `OCS_TEXT_SDF=0` to
+/// fall back to the legacy stroke path (kill switch for the stroke renderer).
 pub fn sdf_text_enabled() -> bool {
     static ON: OnceLock<bool> = OnceLock::new();
-    *ON.get_or_init(|| std::env::var_os("OCS_TEXT_SDF").is_some())
+    *ON.get_or_init(|| std::env::var("OCS_TEXT_SDF").map_or(true, |v| v != "0"))
 }
 
 /// Debug: when env `OCS_TEXT_BOX` is set, each SDF text run also emits a