fix(stretch): free crossing-window corners from Ortho, draw filled marquee (#291)

STRETCH picks its crossing window as two corner points. After the first
corner set last_point, Ortho/Polar constrained the opposite corner to an axis
through it, collapsing the window to a line and making selection impossible.

Add CadCommand::window_corner_pick(); STRETCH returns true for its two window
steps, and the viewport skips the ortho/polar (and OTRACK) lock for that point
in both the preview and commit paths — the corners are now free.

Also render the window like a normal box selection instead of a bare green
outline: add SelectionState::preview_box (a hit-test-free marquee), draw it
with the shared draw_marquee helper (green crossing fill + stroke), and drive
it from the command's new window_first_corner(). Cleared on command end.

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

src/ui/overlay.rs

@@ -430,36 +430,19 @@ impl canvas::Program<Message> for SelectionCanvas {
         }
 
 
-        if let (Some(a), Some(b)) = (self.selection.box_anchor, self.selection.box_current) {
-            let (fill, stroke) = if self.selection.box_crossing {
+        // Draw a selection marquee (green crossing / blue window) as a filled,
+        // stroked rectangle between two canvas points. Shared by the live
+        // box-selection and the preview-only window marquee (#291).
+        fn draw_marquee(frame: &mut canvas::Frame, a: Point, b: Point, crossing: bool) {
+            let (fill, stroke) = if crossing {
                 (
-                    Color {
-                        r: 0.20,
-                        g: 0.72,
-                        b: 0.44,
-                        a: 0.12,
-                    },
-                    Color {
-                        r: 0.20,
-                        g: 0.72,
-                        b: 0.44,
-                        a: 0.9,
-                    },
+                    Color { r: 0.20, g: 0.72, b: 0.44, a: 0.12 },
+                    Color { r: 0.20, g: 0.72, b: 0.44, a: 0.9 },
                 )
             } else {
                 (
-                    Color {
-                        r: 0.20,
-                        g: 0.44,
-                        b: 0.72,
-                        a: 0.12,
-                    },
-                    Color {
-                        r: 0.20,
-                        g: 0.44,
-                        b: 0.72,
-                        a: 0.9,
-                    },
+                    Color { r: 0.20, g: 0.44, b: 0.72, a: 0.12 },
+                    Color { r: 0.20, g: 0.44, b: 0.72, a: 0.9 },
                 )
             };
             let x0 = a.x.min(b.x);
@@ -478,6 +461,14 @@ impl canvas::Program<Message> for SelectionCanvas {
             );
         }
 
+        if let (Some(a), Some(b)) = (self.selection.box_anchor, self.selection.box_current) {
+            draw_marquee(&mut frame, a, b, self.selection.box_crossing);
+        }
+        // Preview marquee for point-picked windows (STRETCH) — same look, no pick.
+        if let Some((a, b, crossing)) = self.selection.preview_box {
+            draw_marquee(&mut frame, a, b, crossing);
+        }
+
         if self.selection.poly_active && self.selection.poly_points.len() > 1 {
             let (fill, stroke) = if self.selection.poly_crossing {
                 (