feat(commandline): fade history entries off the overlay after 3 s

The overlay used to keep the last four history lines on screen
indefinitely, leaving stale command output cluttering the viewport.
Stamp each `HistoryEntry` with its `Instant` of insertion and filter
the rows shown on the overlay to entries younger than 3 s.

`CommandLine::has_visible_history()` gates a per-frame tick
subscription so the view re-renders and the entry disappears
on time. The subscription auto-stops once no entry is fresh enough,
keeping the idle CPU draw at zero (#18).

Full history stays in the backing `Vec` — the upcoming dropdown
button will surface it on demand.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/view.rs

@@ -661,8 +661,19 @@ impl OpenCADStudio {
         } else {
             Subscription::none()
         };
+        // While the command-line overlay is still displaying any
+        // recently-pushed history entry, re-render every frame so the
+        // entry disappears at the moment its visible window expires.
+        // The subscription auto-stops once no entry is fresh enough
+        // (typically within a few seconds of the last command).
+        let history_tick = if self.command_line.has_visible_history() {
+            window::frames().map(Message::Tick)
+        } else {
+            Subscription::none()
+        };
         iced::Subscription::batch([
             frames,
+            history_tick,
             event::listen_with(|ev, status, win_id| {
                 use iced::event::Status;
                 match ev {