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>

docs/render-settings.md

@@ -2,8 +2,10 @@
 
 Named setups save common render intent in the document. They do not start a render,
 apply the navigation camera or make a currently unavailable backend available.
-The Render workspace, scene extraction, assets, PBR materials and jobs are later slices
-of [#620](https://github.com/bherbruck/solidcraft/issues/620).
+The integrated workspace and jobs consume these settings through their own capability
+checks. [Opaque PBR](render-opaque-pbr.md) is an opt-in appearance response; texture
+and environment assets remain follow-ups of [#620](https://github.com/bherbruck/solidcraft/issues/620);
+see [the combined integration boundary](render-integration.md).
 
 ## Commands