viewer: egui depth format 불일치 수정 + 변수명 경고 제거

- egui_wgpu::Renderer:🆕 depth format None (UI는 depth 불필요)
- bridge_scene.rs: SPAN_M 등 snake_case로 전환

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cimery/crates/viewer/src/lib.rs

@@ -284,8 +284,9 @@ impl RenderState {
             &*window,
             None, None, None,
         );
+        // egui renders 2D UI overlay — no depth buffer
         let egui_renderer = egui_wgpu::Renderer::new(
-            &device, format, Some(DEPTH_FORMAT), 1, false,
+            &device, format, None, 1, false,
         );
 
         RenderState {