fix(app): running OSNAP set is app-level, not per-drawing

Modern DWG (R2000+) has no file slot for OSMODE — reading a file left
the header at 0, and adopt_header_sysvars wiped the user's running snap
set on every open. Persist the set (as an $OSMODE-style bitmask) in
settings.json, save it on every snap toggle, and only let a file
override it when it actually carries a nonzero mask (R13/R14 DWG, DXF).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/commands/styleprops.rs

@@ -1309,6 +1309,8 @@ impl OpenCADStudio {
                         );
                         self.snapper.enabled = modes.into_iter().collect();
                         self.snapper.snap_enabled = en;
+                        // The OSNAP set is app-level (no modern-DWG file slot).
+                        self.persist_settings_if_changed();
                     }
                 }
             }

src/app/settings.rs

@@ -116,6 +116,12 @@ pub struct UserSettings {
     /// instead of submitting (as if every line started with `>`), until the
     /// user toggles it back off.
     pub literal_spaces: bool,
+    /// Running object-snap set + master toggle as an `$OSMODE`-style bitmask.
+    /// App-level, not per-drawing: modern DWG (R2000+) has no file slot for
+    /// OSMODE (it moved to the registry), so the set follows the user. A
+    /// legacy R13/R14 or DXF file carrying a nonzero `$OSMODE` still overrides
+    /// it on open (see `adopt_header_sysvars`).
+    pub osmode: i32,
     /// Controls whether the TEXTEDIT command repeats automatically (0 = Multiple, 1 = Single).
     pub texteditmode: bool,
     /// TEXTFILL: fill TrueType glyphs (true) or draw them hollow (false).
@@ -148,6 +154,9 @@ impl Default for UserSettings {
             disabled_plugins: Vec::new(),
             plugin_repos: Vec::new(),
             literal_spaces: false,
+            // Snapper::default(): END|MID|CEN|NODE|QUAD|INT|NEA (575), master
+            // off (suppress bit 16384).
+            osmode: 575 | OSMODE_SUPPRESS,
             texteditmode: false,
             textfill: true,
             backup_on_save: true,

src/app/update/file.rs

@@ -76,6 +76,10 @@ impl OpenCADStudio {
             },
             plugin_repos: self.plugin_repos.clone(),
             literal_spaces: self.command_line.literal_spaces,
+            osmode: crate::app::settings::osmode_from_snaps(
+                self.snapper.enabled.iter(),
+                self.snapper.snap_enabled,
+            ),
             texteditmode: self.texteditmode,
             textfill: crate::scene::text::sdf_atlas::textfill(),
             backup_on_save: self.backup_on_save,
@@ -98,6 +102,9 @@ impl OpenCADStudio {
         self.disabled_plugins = s.disabled_plugins.iter().cloned().collect();
         self.plugin_repos = s.plugin_repos.clone();
         self.command_line.literal_spaces = s.literal_spaces;
+        let (modes, snap_enabled) = crate::app::settings::snaps_from_osmode(s.osmode);
+        self.snapper.enabled = modes.into_iter().collect();
+        self.snapper.snap_enabled = snap_enabled;
         self.texteditmode = s.texteditmode;
         crate::scene::text::sdf_atlas::set_textfill(s.textfill);
         self.backup_on_save = s.backup_on_save;
@@ -132,9 +139,15 @@ impl OpenCADStudio {
         if ortho {
             self.polar_mode = false;
         }
-        let (modes, snap_enabled) = crate::app::settings::snaps_from_osmode(osmode);
+        // OSMODE has no file slot in modern DWG (R2000+ moved it to the
-        self.snapper.enabled = modes.into_iter().collect();
+        // registry), so a header value of 0 just means "absent" — keep the
-        self.snapper.snap_enabled = snap_enabled;
+        // user's app-level set instead of wiping it. Only a legacy R13/R14 or
+        // DXF file that really carries a nonzero mask overrides.
+        if osmode != 0 {
+            let (modes, snap_enabled) = crate::app::settings::snaps_from_osmode(osmode);
+            self.snapper.enabled = modes.into_iter().collect();
+            self.snapper.snap_enabled = snap_enabled;
+        }
     }
 
     /// Stamp the live per-drawing sysvars (Ortho, running OSNAP) onto tab `i`'s

src/app/update/mod.rs

@@ -1548,6 +1548,7 @@ impl OpenCADStudio {
             Message::ToggleSnapEnabled => {
                 self.snapper.toggle_global();
                 self.sync_vport_display(self.active_tab);
+                self.persist_settings_if_changed();
                 Task::none()
             }
             Message::ToggleGridSnap => {
@@ -2048,6 +2049,7 @@ impl OpenCADStudio {
             }
             Message::ToggleSnap(t) => {
                 self.snapper.toggle(t);
+                self.persist_settings_if_changed();
                 Task::none()
             }
             Message::ToggleSnapPopup => {