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/commands/layers.rs

@@ -444,7 +444,9 @@ impl OpenCADStudio {
                         .push_error("ISOLATEOBJECTS: select the objects to isolate first.");
                 } else {
                     let n = self.tabs[i].scene.selected.len();
+                    self.push_undo_snapshot(i, "ISOLATEOBJECTS");
                     self.tabs[i].scene.isolate_selected();
+                    self.tabs[i].dirty = true;
                     self.command_line.push_info(&format!(
                         "Isolated {n} object(s). UNISOLATEOBJECTS to restore."
                     ));
@@ -458,7 +460,9 @@ impl OpenCADStudio {
                         .push_error("HIDEOBJECTS: select the objects to hide first.");
                 } else {
                     let n = self.tabs[i].scene.selected.len();
+                    self.push_undo_snapshot(i, "HIDEOBJECTS");
                     self.tabs[i].scene.hide_selected();
+                    self.tabs[i].dirty = true;
                     self.command_line
                         .push_info(&format!("Hid {n} object(s). UNISOLATEOBJECTS to restore."));
                 }
@@ -467,7 +471,9 @@ impl OpenCADStudio {
             // UNISOLATEOBJECTS — bring back everything hidden by Isolate / Hide
             "UNISOLATEOBJECTS" => {
                 if self.tabs[i].scene.is_isolation_active() {
+                    self.push_undo_snapshot(i, "UNISOLATEOBJECTS");
                     self.tabs[i].scene.end_isolation();
+                    self.tabs[i].dirty = true;
                     self.command_line
                         .push_info("Isolation ended — all objects shown.");
                 } else {

src/app/commands/styleprops.rs

@@ -1250,6 +1250,22 @@ impl OpenCADStudio {
                     if name == "TEXTFILL" {
                         self.tabs[i].scene.bump_geometry();
                     }
+                    // ORTHOMODE / OSMODE set the header directly; mirror them into
+                    // the live Ortho / running OSNAP so the constraint + status
+                    // bar follow and the save-time stamp doesn't revert them.
+                    if name == "ORTHOMODE" {
+                        self.ortho_mode = self.tabs[i].scene.document.header.ortho_mode;
+                        if self.ortho_mode {
+                            self.polar_mode = false;
+                        }
+                    }
+                    if name == "OSMODE" {
+                        let (modes, en) = crate::app::settings::snaps_from_osmode(
+                            self.tabs[i].scene.document.header.object_snap_mode,
+                        );
+                        self.snapper.enabled = modes.into_iter().collect();
+                        self.snapper.snap_enabled = en;
+                    }
                 }
             }