Phase 4: Viewport control and properties

- Viewport ID uniqueness: commit_entity assigns max_existing_id+1
  instead of always using id=2
- Lock/On boolean toggles in Properties panel (vp_locked, vp_on)
  backed by status.locked / status.is_on fields
- Standard scale Choice picker: 1:500 to 10:1 (also syncs view_height)
- Numeric scale edit now syncs view_height when changed
- Standard view Choice picker: Top/Bottom/Front/Back/Left/Right/4×Iso
  updates view_direction vector via apply_geom_prop
- Viewport content not rendered when status.is_on is false
- current_layout_block_handle_pub() added for external access

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/scene/mod.rs

@@ -84,6 +84,13 @@ impl Scene {
         }
     }
 
+    /// Public accessor for the block-record handle of the current layout.
+    /// Used by external callers (e.g. `commit_entity`) that need the handle
+    /// without going through private API.
+    pub fn current_layout_block_handle_pub(&self) -> Handle {
+        self.current_layout_block_handle()
+    }
+
     /// Returns the block-record handle for `current_layout`.
     fn current_layout_block_handle(&self) -> Handle {
         self.document
@@ -382,7 +389,8 @@ impl Scene {
                     None
                 }
             })
-            .filter(|vp| vp.id > 1 && vp.common.owner_handle == paper_block)
+            // Only user viewports (id > 1) that belong to this layout AND are turned on.
+            .filter(|vp| vp.id > 1 && vp.common.owner_handle == paper_block && vp.status.is_on)
             .collect();
 
         if viewports.is_empty() {