fix(select): pin box-selection anchor to the world during zoom/pan (#234)

The window/crossing selection anchor was stored only in screen pixels, so
zooming or panning mid-drag left it frozen while the drawing moved under
it — the rectangle then covered the wrong world area and selected too much
or too little.

Store the anchor's world point and re-project it to the screen anchor
whenever the model-space view zooms (on_viewport_scroll) or pans
(on_viewport_move), so the rectangle tracks the drawing. The moving corner
already follows the cursor. (MSPACE floating-viewport box-select would
need the viewport camera and is left as-is.)

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/pick/selection_state.rs

@@ -6,6 +6,11 @@ use iced::Point;
 pub struct SelectionState {
     pub vp_size: (f32, f32),
     pub box_anchor: Option<Point>,
+    /// World point under the box-selection anchor, so the anchor can be
+    /// re-projected to screen when the camera zooms/pans mid-drag instead of
+    /// staying frozen at its original pixel (which selected the wrong area).
+    /// (#234)
+    pub box_anchor_world: Option<glam::DVec3>,
     pub box_current: Option<Point>,
     pub box_last: Option<(Point, Point)>,
     pub box_crossing: bool,