fix(selection): individual picks accumulate; Shift+click removes

Single-click picking cleared the selection first (exclusive), so clicking a
second object dropped the first (issue #47). Plain clicks now add to the
selection and Shift+click removes the picked entity; empty-space click and
Esc still clear. Shift state is tracked from keyboard ModifiersChanged
events since mouse click messages carry no modifiers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -246,6 +246,10 @@ pub(super) struct OpenCADStudio {
     clipboard: Vec<acadrust::EntityType>,
     /// Centroid of the clipboard entities (XZ plane, Y-up).
     clipboard_centroid: glam::Vec3,
+    /// True while the Shift key is held — drives subtractive pick (Shift+click
+    /// removes the picked entity from the selection). Tracked from keyboard
+    /// modifier-change events since mouse click messages carry no modifiers.
+    shift_down: bool,
     /// Which layout tab has its context menu open (None = closed).
     layout_context_menu: Option<String>,
     /// Inline rename state: (original_name, current_edit_value).
@@ -702,6 +706,9 @@ pub enum Message {
     /// Extend the current selection with every entity in the active
     /// layout that matches a selected entity by (type, layer).
     SelectSimilar,
+    /// Keyboard modifier state changed — tracks whether Shift is held so the
+    /// pick path can do subtractive (Shift+click) selection.
+    SetShiftDown(bool),
     // ── Draw Order context menu ─────────────────────────────────────────
     /// Toggle the Draw Order sub-items in the viewport context menu.
     DrawOrderSubmenuToggle,
@@ -941,6 +948,7 @@ impl OpenCADStudio {
             update_notice_body: None,
             clipboard: Vec::new(),
             clipboard_centroid: glam::Vec3::ZERO,
+            shift_down: false,
             layout_context_menu: None,
             layout_rename_state: None,
             last_vp_click_time: None,

src/app/update.rs

@@ -2782,8 +2782,16 @@ impl OpenCADStudio {
                                     )
                                 });
                             if let Some(handle) = hit {
-                                self.tabs[i].scene.select_entity(handle, true);
-                                self.tabs[i].scene.expand_selection_for_groups(&[handle]);
+                                // Individual picks accumulate (issue #47):
+                                // each plain click adds to the selection,
+                                // Shift+click removes the picked entity.
+                                // Esc / empty-space click clears.
+                                if self.shift_down {
+                                    self.tabs[i].scene.deselect_entity(handle);
+                                } else {
+                                    self.tabs[i].scene.select_entity(handle, false);
+                                    self.tabs[i].scene.expand_selection_for_groups(&[handle]);
+                                }
                                 self.refresh_properties();
                                 selection_just_completed = true;
                             } else {
@@ -3380,6 +3388,11 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::SetShiftDown(down) => {
+                self.shift_down = down;
+                Task::none()
+            }
+
             Message::DrawOrderSubmenuToggle => {
                 let i = self.active_tab;
                 let mut sel = self.tabs[i].scene.selection.borrow_mut();

src/app/view.rs

@@ -995,6 +995,9 @@ impl OpenCADStudio {
                     iced::Event::Window(window::Event::Resized(sz)) => {
                         Some(Message::WindowResized(sz.width as f32, sz.height as f32))
                     }
+                    iced::Event::Keyboard(keyboard::Event::ModifiersChanged(m)) => {
+                        Some(Message::SetShiftDown(m.shift()))
+                    }
                     iced::Event::Keyboard(keyboard::Event::KeyPressed {
                         key, modifiers, text, ..
                     }) => {

src/scene/mod.rs

@@ -3808,6 +3808,13 @@ impl Scene {
         self.bump_geometry();
     }
 
+    /// Remove a single entity from the selection (Shift+click subtractive pick).
+    pub fn deselect_entity(&mut self, handle: Handle) {
+        if self.selected.remove(&handle) {
+            self.bump_geometry();
+        }
+    }
+
     pub fn selected_entities(&self) -> Vec<(Handle, &EntityType)> {
         self.selected
             .iter()