feat(dyn): locked input fields constrain the preview and the click (#356)

A typed dynamic-input value (TAB-locked angle or distance) now shapes
the rubber-band: the cursor point is routed through dyn_resolve_point —
the same resolution Enter commits — in the mouse-move preview and the
click commit, so a locked 30° pins the line's direction while the free
field keeps tracking the cursor. The lock wins over osnap/ortho/polar.
TAB (and the comma advance) reshape the preview immediately instead of
waiting for the next mouse move.

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

src/app/update/mod.rs

@@ -799,6 +799,9 @@ impl OpenCADStudio {
                 let n = self.tabs[i].dyn_fields.len();
                 if n > 0 {
                     self.tabs[i].dyn_active = (self.tabs[i].dyn_active + 1) % n;
+                    // TAB locks the value just typed — reshape the rubber-band
+                    // to the constrained point now (#356).
+                    self.refresh_active_cmd_preview(i);
                 }
                 self.focus_cmd_input()
             }