Fix: viewport MSPACE activation and navigation

Four bugs prevented viewports from working like AutoCAD model-space windows:

1. Dead-code double-click block: `!is_down2` was always false because
   `is_down2 = sel.left_down` is captured while the button is still held
   (before the selection block clears it). Changed to `is_down` so the
   double-click handler actually fires.

2. Inverted pan direction: pan_active_viewport used `-=` on the delta
   returned by screen_delta_to_world, which is the same delta that cam.pan
   *adds* to its target — causing reversed drag. Changed to `+=`.

3. Zoom not cursor-centered: zoom_active_viewport now accepts an optional
   paper-space cursor position and adjusts view_target so the model point
   under the cursor stays fixed (same as cam.zoom_about_point).

4. ESC did not exit MSPACE: CommandEscape now calls ExitViewport when
   active_viewport is set and no command is running.

Also added MS/MSPACE and PSPACE command aliases, and MspaceCommand /
PspaceCommand messages so MS auto-enters the first viewport.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/mod.rs

@@ -224,6 +224,10 @@ pub enum Message {
     EnterViewport(acadrust::Handle),
     /// Exit MSPACE and return to paper-space editing (PSPACE).
     ExitViewport,
+    /// MS command: enter MSPACE for the first available viewport.
+    MspaceCommand,
+    /// PS command: exit MSPACE (PSPACE).
+    PspaceCommand,
     /// Switch to a named layout ("Model" or paper space layout name).
     LayoutSwitch(String),
     /// Create a new paper space layout.