feat: editable grid spacing and major grid lines in Drafting Settings

The Snap and Grid tab showed static grid values with dead checkboxes, and every grid line stroked identically.

Grid X/Y spacing and Major-line-every are now editable inputs (validated 2-100 for majors, dirty-tracked, applied with an error message on bad input); Adaptive-grid and Display-beyond-Limits checkboxes are wired to live state. Every Nth line partitions into major_segments and strokes subtly brighter (1.5x alpha, 0.75px) for the reference hierarchy. GRIDMAJOR state persists to UserSettings. Covers partition, invalidation, and parse tests.

src/app/update/file.rs

@@ -533,6 +533,7 @@ impl OpenCADStudio {
             snap_spacing_y: self.snapper.snap_spacing_y,
             grid_spacing_x: self.grid_spacing_x,
             grid_spacing_y: self.grid_spacing_y,
+            grid_major_every: self.grid_major_every,
             grid_adaptive: self.grid_adaptive,
             grid_beyond_limits: self.grid_beyond_limits,
             block_mru: self.block_mru.clone(),
@@ -641,6 +642,7 @@ impl OpenCADStudio {
             crate::app::settings::sanitize_snap_spacing(s.snap_spacing_y);
         self.grid_spacing_x = crate::app::settings::sanitize_snap_spacing(s.grid_spacing_x);
         self.grid_spacing_y = crate::app::settings::sanitize_snap_spacing(s.grid_spacing_y);
+        self.grid_major_every = crate::app::settings::sanitize_grid_major(s.grid_major_every);
         self.grid_adaptive = s.grid_adaptive;
         self.grid_beyond_limits = s.grid_beyond_limits;
         // Block usage: clone but cap to sane sizes (MRU 20, freq map 200)

src/app/update/mod.rs

@@ -4596,6 +4596,36 @@ impl OpenCADStudio {
                 }
                 Task::none()
             }
+            Message::DraftingSettingsGridXChanged(value) => {
+                if let Some(state) = &mut self.drafting_settings_state {
+                    state.grid_x_input = value;
+                }
+                Task::none()
+            }
+            Message::DraftingSettingsGridYChanged(value) => {
+                if let Some(state) = &mut self.drafting_settings_state {
+                    state.grid_y_input = value;
+                }
+                Task::none()
+            }
+            Message::DraftingSettingsGridMajorChanged(value) => {
+                if let Some(state) = &mut self.drafting_settings_state {
+                    state.grid_major_input = value;
+                }
+                Task::none()
+            }
+            Message::DraftingSettingsToggleAdaptiveGrid => {
+                if let Some(state) = &mut self.drafting_settings_state {
+                    state.grid_adaptive = !state.grid_adaptive;
+                }
+                Task::none()
+            }
+            Message::DraftingSettingsToggleBeyondLimits => {
+                if let Some(state) = &mut self.drafting_settings_state {
+                    state.grid_beyond_limits = !state.grid_beyond_limits;
+                }
+                Task::none()
+            }
             Message::DraftingSettingsToggleEqualSnap => {
                 if let Some(state) = &mut self.drafting_settings_state {
                     state.snap_equal = !state.snap_equal;