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/mod.rs

@@ -2741,6 +2741,9 @@ pub enum Message {
     DraftingSettingsTabChanged(crate::ui::window::drafting_settings::DraftingSettingsTab),
     DraftingSettingsToggleGrid,
     DraftingSettingsToggleSnap,
+    DraftingSettingsSnapXChanged(String),
+    DraftingSettingsSnapYChanged(String),
+    DraftingSettingsToggleEqualSnap,
     DraftingSettingsToggleIsometric,
     DraftingSettingsSetIsoPlane(crate::app::settings::IsoPlane),
     DraftingSettingsResetRotation,