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/ui-egui/src/picker_test_support.rs

@@ -0,0 +1,25 @@
+//! Controlled host futures shared by Render and library picker regressions.
+use crate::{SolveApp, file_picker::FilePickerFuture};
+use std::{
+    cell::RefCell,
+    future::Future,
+    pin::Pin,
+    rc::Rc,
+    task::{Context, Poll},
+};
+pub type Slot = Rc<RefCell<Option<Option<String>>>>;
+struct Manual(Slot);
+impl Future for Manual {
+    type Output = Option<String>;
+    fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Self::Output> {
+        self.0.borrow_mut().take().map_or(Poll::Pending, Poll::Ready)
+    }
+}
+pub fn save(app: &mut SolveApp, slot: &Slot) {
+    let slot = slot.clone();
+    app.services.pick_save = Some(Box::new(move |_, _| Box::pin(Manual(slot.clone())) as FilePickerFuture));
+}
+pub fn open(app: &mut SolveApp, slot: &Slot) {
+    let slot = slot.clone();
+    app.services.pick_open = Some(Box::new(move || Box::pin(Manual(slot.clone())) as FilePickerFuture));
+}