refactor(renderer): activate depth buffer on all render paths (Étape 9)

- 9.1: allocate shared Depth32Float depth texture + view in Renderer,
  sized to the initial surface (create_depth_texture helper, reusable
  for the Phase 4.4 resize); Renderer::new now takes width/height.
- 9.2: attach depth_stencil_attachment (clear 1.0 / store) to both the
  low-level render and high-level render_scene passes.
- 9.3: every pipeline declares a matching DepthStencilState (write true,
  compare Less) via the shared DEPTH_FORMAT constant (D1).

Décisions D1-D4 actées 2026-09-18 (DRAFT Étape 9).

lib/examples/manual.rs

@@ -64,7 +64,7 @@ impl ApplicationHandler for App {
 
         // Rendu 2D plat : `standard` en mode unlit (les bind groups frame+object sont posés par
         // draw_entity, la matrice frame par défaut est l'identité → positions NDC inchangées).
-        let mut renderer = Renderer::new(&context, format);
+        let mut renderer = Renderer::new(&context, format, 800, 600);
         renderer.set_unlit(true);
 
         // 3. Material : On utilise renderer.device() et renderer.format()