Feat: viewport display lock, extended properties, VPLAYER and VPORTS commands

- Enforce status.locked in pan_active_viewport and zoom_active_viewport:
  locked viewports now reject pan/zoom inputs entirely
- Add perspective, render_mode, shade_plot_mode, hide_plot, ucs_icon_visible
  fields to the viewport property panel with appropriate pickers/toggles
- Add VPORTS command: lists all user viewports in the current layout
  (id, size, position, scale, on/locked state) in the command line
- Add VPLAYER command: per-viewport layer freeze/thaw from the command line
  (F <layer> to freeze, T <layer> to thaw, Enter to exit)
- Add CmdResult::VpLayerUpdate variant and its handler in apply_cmd_result

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

src/command/mod.rs

@@ -83,6 +83,12 @@ pub enum CmdResult {
     CreateGroup { handles: Vec<Handle>, name: String },
     /// Dissolve all groups that contain any of the given handles; end command.
     DeleteGroups { handles: Vec<Handle> },
+    /// Freeze or thaw layers by name in the given viewport; command stays active.
+    VpLayerUpdate {
+        vp_handle: Handle,
+        freeze: Vec<String>,
+        thaw: Vec<String>,
+    },
     /// Paste clipboard entities translated so their centroid lands at `base_pt`; end command.
     PasteClipboard { base_pt: Vec3 },
 }