Render: integrated workspace, preview image jobs and opaque PBR (#1046)

Lands the Render workspace (R3), bounded preview-image jobs with cancellation and atomic PNG export (R4a), and opaque PBR looks as one change.

- Render workspace: an overlay-free product preview on the installed GPU device. Returning to Design restores its camera. render.scene and render.settings.apply are added.
- Image jobs: render.preflight/start/status/cancel/export run bounded CPU preview jobs. Export captures the job, overwrite choice and document, and checks the caller's file scope before writing.
- Opaque PBR: looks can carry metal fraction, roughness and dielectric IOR. The product viewport and preview PNG use them; geometry and mass are unchanged. The v1 catalog export refuses PBR rather than dropping it.
- Render setups and PBR looks are saved under design format 21. Labels are in en/de/ja/ko/zh/ru.

Supersedes #706 and #800. Refs #620, #673, #756, #1004.

Contributed by @0xlillith.

Co-authored-by: lillith <35337584+0xlillith@users.noreply.github.com>

crates/mcp/src/backend.rs

@@ -172,7 +172,15 @@ impl Default for Headless {
 }
 
 impl Headless {
-    pub fn new(session: Session) -> Self {
+    pub fn new(mut session: Session) -> Self {
+        match solidcraft_engine::render::job::CpuHost::for_platform() {
+            Ok(host) => {
+                if let Err(e) = session.set_render_job_host(Some(host)) {
+                    session.log.push(format!("image host unavailable: {e}"));
+                }
+            }
+            Err(e) => session.log.push(format!("image host unavailable: {e}")),
+        }
         Headless { session, view: StandardView::Iso }
     }
 }