ucs: dynamic UCS on planar solid faces

With dynamic UCS on (UCSDETECT 1, toggled with F6), a command's first
point picked on a planar face of a solid sets the UCS to that face for
the rest of the command, so 2D commands draw on the face (#1711). An
unsnapped first point lands on the face itself; a snapped point off the
face's plane belongs to other geometry and leaves the UCS alone. The
previous UCS comes back when the command ends. UCS commands themselves
are left out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/command_driver/utilities.rs

@@ -624,6 +624,48 @@ impl OpenCADStudio {
         Some(key)
     }
 
+    /// Dynamic UCS: a command's first point picked on a planar solid face sets
+    /// the UCS to that face for the rest of the command. Returns the picked
+    /// point on the face. A snapped point (`snapped`) off the face's plane
+    /// belongs to other geometry, so it leaves the UCS alone.
+    pub(in crate::app) fn apply_dynamic_ucs(
+        &mut self,
+        snapped: Option<glam::DVec3>,
+    ) -> Option<glam::DVec3> {
+        let i = self.active_tab;
+        if !self.dynamic_ucs
+            || self.dynamic_ucs_restore.is_some()
+            || self.last_point.is_some()
+            || !self.tabs[i].editing_model_space()
+            || self.tabs[i]
+                .active_cmd
+                .as_ref()
+                .is_none_or(|command| command.name().starts_with("UCS"))
+        {
+            return None;
+        }
+        let cursor = self.tabs[i].scene.selection.borrow().input.last_move_pos?;
+        let (vw, vh) = self.tabs[i].scene.selection.borrow().view.vp_size;
+        let bounds = self.tabs[i].scene.active_model_tile_bounds(vw, vh);
+        let local = iced::Point::new(cursor.x - bounds.x, cursor.y - bounds.y);
+        let (ucs, pick) = self.tabs[i].scene.planar_face_ucs_at(bounds, local)?;
+        if let Some(point) = snapped {
+            let normal = glam::DVec3::new(ucs.x_axis.x, ucs.x_axis.y, ucs.x_axis.z)
+                .cross(glam::DVec3::new(ucs.y_axis.x, ucs.y_axis.y, ucs.y_axis.z))
+                .normalize_or_zero();
+            let off_plane = (point - pick).dot(normal).abs();
+            if off_plane > 1.0e-6 * (1.0 + point.length().max(pick.length())) {
+                return None;
+            }
+        }
+        let saved = self.tabs[i].active_ucs.take();
+        self.dynamic_ucs_restore = Some((self.tabs[i].id, saved));
+        self.tabs[i].active_ucs = Some(ucs);
+        self.tabs[i].sync_ucs_to_scene();
+        self.push_ucs_to_cmd(i);
+        Some(pick)
+    }
+
     /// `feed_command`, also reporting whether the step actually took the input.
     /// Only a `Text` token can come back unclaimed (`on_text_input` returning
     /// `None`), which is what lets the caller read it as something else — a
@@ -632,6 +674,9 @@ impl OpenCADStudio {
         &mut self,
         input: StepInput,
     ) -> (Task<Message>, bool) {
+        if matches!(input, StepInput::Point(_)) {
+            let _ = self.apply_dynamic_ucs(None);
+        }
         let gathering = self.report_selection_found(&input);
         // A command that takes associative dimensions only keeps just those:
         // the rest leave the selection and the count of what went is reported,