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/mod.rs

@@ -257,6 +257,7 @@ fn command_kind_label(cmd: &rdbms_playground::dsl::Command) -> String {
             AppCommand::Messages { .. } => "App(Messages)".into(),
             AppCommand::Undo => "App(Undo)".into(),
             AppCommand::Redo => "App(Redo)".into(),
+            AppCommand::Copy { .. } => "App(Copy)".into(),
         },
     }
 }