feat(snap): Shift+RMB one-shot snap override menu (#337)

Shift+right-click in the viewport opens a cursor-anchored grid of snap
icons (names as hover tooltips). Picking one engages a one-shot
override: only that snap applies — even with running osnap off — until
the next point pick commits, which restores the saved configuration.
Esc drops an unconsumed override; an outside click closes the menu.
The status-bar snap toggles are untouched — this is a separate,
temporary mechanism.

Covers items 1 and 2 of #337 (mid-between-two-points still pending).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/update/command.rs

@@ -431,6 +431,10 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
     }
 
     pub(super) fn on_command_escape(&mut self) -> Task<Message> {
+                // Esc drops an unconsumed one-shot snap override and closes
+                // its menu (#337). Falls through — Esc keeps its usual effect.
+                self.snap_override_popup = None;
+                self.snapper.clear_override();
                 // Open MText editor swallows Escape (cancel without committing).
                 if self.mtext_editor.is_some() {
                     self.mtext_cancel();

src/app/update/mod.rs

@@ -292,6 +292,24 @@ impl OpenCADStudio {
                 }
             }
 
+            Message::SnapOverridePick(t) => {
+                self.snap_override_popup = None;
+                self.snapper.set_override(t);
+                let label = crate::snap::ALL_SNAP_MODES
+                    .iter()
+                    .find(|(m, _, _)| *m == t)
+                    .map(|(_, _, l)| *l)
+                    .unwrap_or("Snap");
+                self.command_line
+                    .push_info(&format!("Snap override: {label} (next pick only)."));
+                Task::none()
+            }
+
+            Message::SnapOverrideClose => {
+                self.snap_override_popup = None;
+                Task::none()
+            }
+
             Message::FileDropped(path) => {
                 // Desktop drag & drop (#344): accept the formats the Open
                 // dialog accepts — a drop has no picker filter, so anything
@@ -1367,6 +1385,15 @@ impl OpenCADStudio {
             Message::ViewportRightPress => {
                 let i = self.active_tab;
                 self.ribbon.close_dropdown();
+                // Shift+RMB: the one-shot snap override menu at the cursor —
+                // pick a snap for just the next point, then it expires (#337).
+                if self.shift_down {
+                    let pos = self.tabs[i].scene.selection.borrow().last_move_pos;
+                    if let Some(p) = pos {
+                        self.snap_override_popup = Some(p);
+                    }
+                    return Task::none();
+                }
                 let mut sel = self.tabs[i].scene.selection.borrow_mut();
                 let Some(p) = sel.last_move_pos else {
                     return Task::none();

src/app/update/viewport.rs

@@ -2175,6 +2175,9 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                             }
                         }
                         self.last_point = Some(world_pt);
+                        // The one-shot snap override is spent by this pick —
+                        // restore the running osnap configuration (#337).
+                        self.snapper.clear_override();
                         self.dyn_user_reshaped = false;
                         self.sync_dyn_fields();
                         self.reset_tracking_after_point();