feat(snap): editable Snap X/Y spacing with Equal lock, grid snap locks

Drafting Settings dialog gets text inputs for Snap X/Y spacing plus a working Equal X and Y checkbox (mirrored inputs, Y follows X on apply). Snapper gains fixed snap_spacing_x/y (persisted in UserSettings, default 10) decoupled from the adaptive visible-grid step. Grid snap no longer aperture-gated: with Snap On it rounds every point to the grid, object snaps still override by tier. Adds parse/lock regression tests.

src/app/update/file.rs

@@ -443,6 +443,8 @@ impl OpenCADStudio {
             commandline_fade_ms: crate::app::settings::clamp_commandline_fade_ms(
                 self.commandline_fade_ms,
             ),
+            snap_spacing_x: self.snapper.snap_spacing_x,
+            snap_spacing_y: self.snapper.snap_spacing_y,
             block_mru: self.block_mru.clone(),
             block_freq: self.block_freq.clone(),
         }
@@ -540,6 +542,10 @@ impl OpenCADStudio {
             crate::app::settings::clamp_commandline_fade_ms(s.commandline_fade_ms);
         self.command_line
             .set_commandline_fade_ms(self.commandline_fade_ms.clamp(0, 60000) as u32);
+        self.snapper.snap_spacing_x =
+            crate::app::settings::sanitize_snap_spacing(s.snap_spacing_x);
+        self.snapper.snap_spacing_y =
+            crate::app::settings::sanitize_snap_spacing(s.snap_spacing_y);
         // Block usage: clone but cap to sane sizes (MRU 20, freq map 200)
         self.block_mru = s.block_mru.iter().take(20).cloned().collect();
         self.block_freq = s

src/app/update/mod.rs

@@ -4484,6 +4484,33 @@ impl OpenCADStudio {
                 }
                 Task::none()
             }
+            Message::DraftingSettingsSnapXChanged(value) => {
+                if let Some(state) = &mut self.drafting_settings_state {
+                    state.snap_x_input = value.clone();
+                    if state.snap_equal {
+                        state.snap_y_input = value;
+                    }
+                }
+                Task::none()
+            }
+            Message::DraftingSettingsSnapYChanged(value) => {
+                if let Some(state) = &mut self.drafting_settings_state {
+                    state.snap_y_input = value.clone();
+                    if state.snap_equal {
+                        state.snap_x_input = value;
+                    }
+                }
+                Task::none()
+            }
+            Message::DraftingSettingsToggleEqualSnap => {
+                if let Some(state) = &mut self.drafting_settings_state {
+                    state.snap_equal = !state.snap_equal;
+                    if state.snap_equal {
+                        state.snap_y_input = state.snap_x_input.clone();
+                    }
+                }
+                Task::none()
+            }
             Message::DraftingSettingsToggleIsometric => {
                 if let Some(state) = &mut self.drafting_settings_state {
                     state.isometric = !state.isometric;
@@ -4574,17 +4601,19 @@ impl OpenCADStudio {
                 Task::none()
             }
             Message::DraftingSettingsApply => {
-                self.apply_drafting_settings();
-                self.drafting_settings_saved = self.drafting_settings_state.clone();
-                self.persist_settings_if_changed();
+                if self.apply_drafting_settings() {
+                    self.drafting_settings_saved = self.drafting_settings_state.clone();
+                    self.persist_settings_if_changed();
+                }
                 Task::none()
             }
             Message::DraftingSettingsOk => {
-                self.apply_drafting_settings();
-                self.drafting_settings_saved = self.drafting_settings_state.clone();
-                self.drafting_settings_close_confirm = false;
-                self.persist_settings_if_changed();
-                self.close_active_modal();
+                if self.apply_drafting_settings() {
+                    self.drafting_settings_saved = self.drafting_settings_state.clone();
+                    self.drafting_settings_close_confirm = false;
+                    self.persist_settings_if_changed();
+                    self.close_active_modal();
+                }
                 Task::none()
             }
             Message::DraftingSettingsClose => {

src/app/update/viewport.rs

@@ -1928,7 +1928,9 @@ impl OpenCADStudio {
                     )
                 }
             };
-            // Sync grid-snap spacing to the adaptive spacing of the visible grid.
+            // Sync the adaptive visible-grid step. It drives display and
+            // snap-aperture tolerances — grid *snap* itself locks to the
+            // fixed SNAPUNIT X/Y spacing from the Drafting Settings dialog.
             self.snapper.grid_spacing = grid_spacing;
             // Cursor and wires are model-space; the snap result is model.
             let snap_cursor = cursor_world;