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/snapshots/typing_surface_matrix__typing_surface__app_commands__copy_all_parses@copy_all.snap

@@ -0,0 +1,20 @@
+---
+source: tests/typing_surface/app_commands.rs
+assertion_line: 146
+description: "input=\"copy all\" cursor=8"
+expression: "& a"
+---
+Assessment {
+    input: "copy all",
+    cursor: 8,
+    state: Valid,
+    hint: Some(
+        Prose(
+            "Submit with Enter",
+        ),
+    ),
+    completion: None,
+    parse_result: Ok(
+        "App(Copy)",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__app_commands__copy_last_parses@copy_last.snap

@@ -0,0 +1,20 @@
+---
+source: tests/typing_surface/app_commands.rs
+assertion_line: 154
+description: "input=\"copy last\" cursor=9"
+expression: "& a"
+---
+Assessment {
+    input: "copy last",
+    cursor: 9,
+    state: Valid,
+    hint: Some(
+        Prose(
+            "Submit with Enter",
+        ),
+    ),
+    completion: None,
+    parse_result: Ok(
+        "App(Copy)",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__app_commands__copy_space_offers_all_and_last@copy_space.snap

@@ -0,0 +1,52 @@
+---
+source: tests/typing_surface/app_commands.rs
+assertion_line: 161
+description: "input=\"copy \" cursor=5"
+expression: "& a"
+---
+Assessment {
+    input: "copy ",
+    cursor: 5,
+    state: Valid,
+    hint: Some(
+        Candidates {
+            items: [
+                Candidate {
+                    text: "all",
+                    kind: Keyword,
+                    mode: Both,
+                },
+                Candidate {
+                    text: "last",
+                    kind: Keyword,
+                    mode: Both,
+                },
+            ],
+            selected: None,
+        },
+    ),
+    completion: Some(
+        Completion {
+            replaced_range: (
+                5,
+                5,
+            ),
+            partial_prefix: "",
+            candidates: [
+                Candidate {
+                    text: "all",
+                    kind: Keyword,
+                    mode: Both,
+                },
+                Candidate {
+                    text: "last",
+                    kind: Keyword,
+                    mode: Both,
+                },
+            ],
+        },
+    ),
+    parse_result: Ok(
+        "App(Copy)",
+    ),
+}

tests/typing_surface/snapshots/typing_surface_matrix__typing_surface__app_commands__copy_with_no_value_parses@copy_no_value.snap

@@ -0,0 +1,20 @@
+---
+source: tests/typing_surface/app_commands.rs
+assertion_line: 138
+description: "input=\"copy\" cursor=4"
+expression: "& a"
+---
+Assessment {
+    input: "copy",
+    cursor: 4,
+    state: Valid,
+    hint: Some(
+        Prose(
+            "Submit with Enter",
+        ),
+    ),
+    completion: None,
+    parse_result: Ok(
+        "App(Copy)",
+    ),
+}