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.

tests/typing_surface/app_commands.rs

@@ -129,6 +129,38 @@ fn messages_with_value_parses() {
     crate::snap!("messages_verbose", a);
 }
 
+#[test]
+fn copy_with_no_value_parses() {
+    // Copy scope is optional — bare `copy` copies the whole panel.
+    let a = assess_at_end("copy", &schema_empty());
+    assert!(matches!(a.state, InputState::Valid));
+    assert_eq!(a.parse_result.as_deref(), Ok("App(Copy)"));
+    crate::snap!("copy_no_value", a);
+}
+
+#[test]
+fn copy_all_parses() {
+    let a = assess_at_end("copy all", &schema_empty());
+    assert!(matches!(a.state, InputState::Valid));
+    assert_eq!(a.parse_result.as_deref(), Ok("App(Copy)"));
+    crate::snap!("copy_all", a);
+}
+
+#[test]
+fn copy_last_parses() {
+    let a = assess_at_end("copy last", &schema_empty());
+    assert!(matches!(a.state, InputState::Valid));
+    assert_eq!(a.parse_result.as_deref(), Ok("App(Copy)"));
+    crate::snap!("copy_last", a);
+}
+
+#[test]
+fn copy_space_offers_all_and_last() {
+    let a = assess_at_end("copy ", &schema_empty());
+    assert_candidate_present(&a, &["all", "last"]);
+    crate::snap!("copy_space", a);
+}
+
 #[test]
 fn partial_entry_word_classifies_as_definite_error_but_completes() {
     // `qu` — mid-typing the `quit` entry word.