feat(sysvars): persist Ortho, running OSNAP and Isolate in the drawing

Ortho and the running object-snap set were app-global (settings.txt only) and
Isolate/Hide was a session-only view state, so none survived save/reopen and
none were per-drawing. Bind all three to the file like AutoCAD:

- Ortho -> header.ortho_mode ($ORTHOMODE); running OSNAP <-> header
  .object_snap_mode ($OSMODE bitmask) via a new SnapType<->bit map in settings
  (osmode_from_snaps / snaps_from_osmode, round-trip tested). The header is the
  per-drawing store, live state mirrors the active drawing: adopt header->live
  on open and tab-switch, stamp live->header on save and when leaving a tab, a
  fresh drawing inherits the current values, and SETVAR ORTHOMODE/OSMODE now
  also update the live state. settings.txt stays as the boot/new-drawing
  default; the per-file header overrides it on open.
- Isolate/Hide now set each entity's persisted invisible flag (DXF code 60,
  which acadrust round-trips in DXF and DWG) in addition to the session set;
  End Isolation clears it. The set is reseeded from invisible entities on load
  and after undo/redo, the commands are undoable and mark the tab dirty. The
  renderer already skipped invisible entities, so no render change was needed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/history.rs

@@ -45,6 +45,9 @@ impl OpenCADStudio {
         self.tabs[i].scene.populate_hatches_from_document();
         self.tabs[i].scene.populate_images_from_document();
         self.tabs[i].scene.populate_meshes_from_document();
+        // Keep the Isolate/Hide set in step with the restored per-entity
+        // visibility so End Isolation stays correct after undo/redo.
+        self.tabs[i].scene.sync_hidden_from_invisible();
         self.tabs[i].scene.clear_preview_wire();
         self.tabs[i].scene.images.clear();
         self.tabs[i].active_cmd = None;