feat: copy the output panel to the system clipboard (#11)

New app-level `copy` / `copy all` / `copy last` command (ADR-0041).
Delivery is OSC 52 *and* a best-effort native write (arboard), always
both — OSC 52 acceptance is undetectable, so a true fallback can't be
built. Payload is the panel's plain text exactly as rendered (tags,
✓/✗, box-drawing), drift-locked to render_output_line. arboard added
--no-default-features (X11-only; OSC 52 covers Wayland).

Amends ADR-0003's command registry; requirements V6.

src/runtime.rs

@@ -376,6 +376,12 @@ async fn run_loop(
     // no wake-ups. See `IndicatorDebounce` for the decision
     // logic; `app.input_indicator` mirrors it for the renderer.
     let mut debounce = IndicatorDebounce::default();
+    // Long-lived native clipboard for the `copy` command (ADR-0041).
+    // Created lazily on first copy (so an OSC-52-only session never
+    // opens an X11 connection) and kept alive for the session — the
+    // X11 backend serves the selection from a thread owned by this
+    // handle, so it must outlive each write.
+    let mut native_clipboard = crate::clipboard::SystemClipboard::new();
     loop {
         let event = if debounce.is_armed() {
             match tokio::time::timeout(INDICATOR_DEBOUNCE, event_rx.recv()).await {
@@ -557,6 +563,20 @@ async fn run_loop(
                         tracing::warn!(error = %e, "could not persist input mode");
                     }
                 }
+                Action::CopyToClipboard(text) => {
+                    // OSC 52 to the terminal + a best-effort native
+                    // write (ADR-0041). Both always fire; an OSC 52
+                    // write error is logged but never fatal.
+                    let mut out = std::io::stdout();
+                    if let Err(e) = crate::clipboard::deliver(
+                        &mut out,
+                        &mut native_clipboard,
+                        &text,
+                        crate::clipboard::in_tmux(),
+                    ) {
+                        tracing::warn!(error = %e, "could not emit clipboard OSC 52 escape");
+                    }
+                }
             }
         }
         // A keystroke hides the indicator and re-arms the