Render R2: persist named render setups without re-evaluating geometry (#667)

render.settings.set saves validated named render setups (camera, output size, exposure, environment) as one undo step, and render.settings.get reads them back without applying a camera. When validated render settings are the only change, the engine keeps the evaluated model instead of rebuilding it. Setups are saved under design format 21; format-20 designs read as having none. Fixes #658; refs #620.

Contributed by @0xlillith.

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

crates/engine/src/cmd/edit.rs

@@ -14,9 +14,11 @@ pub static COMMANDS: &[CommandSpec] = &[
     CommandSpec::new("edit.redo", "Redo", redo).icon("redo").key("Ctrl+Y").noundo().doc_only(),
     CommandSpec::new("edit.checkpoint", "Checkpoint", checkpoint)
         .noundo()
+        .doc_only()
         .params("→ {checkpoint: id}: remember the design, active sketch and undo history (restore with edit.restore_checkpoint)"),
     CommandSpec::new("edit.restore_checkpoint", "Restore Checkpoint", restore_checkpoint)
         .noundo()
+        .doc_only()
         .params("checkpoint: id; forget?: bool (true = drop it without going back). Goes back to it, undo history included"),
     CommandSpec::new("timeline.compute_all", "Compute All", compute_all).at("SOLID", "MODIFY").icon("compute").noundo(),
     CommandSpec::new("timeline.delete", "Delete", delete)