fix: honor GRIDUNIT X/Y spacing and LIMITS option in grid display

The viewport grid ignored the configured spacing and forced square grids: compute_grid_step hardcoded a 1.0 base, a single shared step drove both line families (including the tangent derivative used for gap measurement), and the display always clipped to LIMITS despite the beyond-Limits option.

Grid spacing now scales per-axis from GRIDUNIT via compute_grid_steps (adaptive 5x growth, fixed passthrough), GridParams carries step_x/step_y, snap tolerances follow the visible steps, and the LIMITS clip is skipped when beyond-Limits is set. Also fixes a per-axis overflow fallback that returned the X base for Y. Spacing state (X/Y, adaptive, beyond-Limits) is stored on the app and persisted to UserSettings at defaults; dialog wiring follows separately.

src/app/update/file.rs

@@ -531,6 +531,10 @@ impl OpenCADStudio {
             ),
             snap_spacing_x: self.snapper.snap_spacing_x,
             snap_spacing_y: self.snapper.snap_spacing_y,
+            grid_spacing_x: self.grid_spacing_x,
+            grid_spacing_y: self.grid_spacing_y,
+            grid_adaptive: self.grid_adaptive,
+            grid_beyond_limits: self.grid_beyond_limits,
             block_mru: self.block_mru.clone(),
             block_freq: self.block_freq.clone(),
         }
@@ -635,6 +639,10 @@ impl OpenCADStudio {
             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);
+        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_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)
         self.block_mru = s.block_mru.iter().take(20).cloned().collect();
         self.block_freq = s