test(protocol): move the version ladder pins to v21

Both pins failed on the bump, which is what they exist for. The ladder
test now accepts 6..=21 and rejects 22, and the version assertion carries
the Stage 2 entry: four variants appended after their enum's final v20
variant, gated in both directions.

Also renames `protocol_version_is_twenty_for_gpu_initial_targets`, whose
name pinned the old number.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RuhVYUPHXMHG8r2z4tsDPR

pmacs-protocol/src/wire_grid.rs

@@ -60,7 +60,7 @@ pub struct WireGridLimits {
 ///
 /// Callers map these onto their own message-specific error types so
 /// existing wire errors keep their exact variants and text.
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub enum WireGridError {
     /// Rows or columns are zero or above this grid's bounds.
     Size {

src/protocol.rs

@@ -1683,7 +1683,7 @@ mod tests {
     // --- M5.5a handshake & postcard round-trips ---
 
     #[test]
-    fn protocol_version_is_twenty_for_gpu_initial_targets() {
+    fn protocol_version_is_twenty_one_for_the_bottom_panel_band() {
         // Pin the value: T M10.5 bumped 1→2 (v1.0 wire: CrdtOp /
         // PresenceUpdate). T M11.1 bumped 2→3 (v1.1 wire: the
         // SemanticFrame family + FrontendEvent::Viewport). T M11.6
@@ -1722,7 +1722,13 @@ mod tests {
         // variant, see the placement pins).
         // GPU initial targets bump 19→20 with a semantic-only
         // SessionBootstrapRequest and appended InitialTargetResult.
-        assert_eq!(PROTOCOL_VERSION, 20);
+        // Bottom panel Stage 2 bumps 20→21 (`InstanceMessage::PanelFrame`,
+        // daemon-gated, plus `FrontendEvent::{FrontendCellGeometry,
+        // PanelResizeRows, PanelPointer}`, frontend-gated — the second
+        // bump that gates in BOTH directions; all four appended after
+        // their enum's final v20 variant, see the placement pins in
+        // `bottom_panel_stage2b_protocol_acceptance`).
+        assert_eq!(PROTOCOL_VERSION, 21);
     }
 
     #[test]
@@ -1798,17 +1804,18 @@ mod tests {
         // minibuffer), v13 (`LineNumbers`), v14 (`LineNumberMode`), v15
         // (`CompletionPopup`), v16 (`ThemeFacts`), v17 (`FontFacts`),
         // v18 (`StatuslineSegments`), v19 (the vterm terminal family),
-        // and v20 (semantic initial-target bootstrap) all interoperate.
-        for accepted in 6..=20 {
+        // v20 (semantic initial-target bootstrap), and v21 (the bottom
+        // panel band) all interoperate.
+        for accepted in 6..=21 {
             assert!(
                 is_supported_protocol_version(accepted),
                 "v{accepted} must be accepted"
             );
         }
-        for rejected in [0, 1, 2, 3, 4, 5, 21, u32::MAX] {
+        for rejected in [0, 1, 2, 3, 4, 5, 22, u32::MAX] {
             assert!(
                 !is_supported_protocol_version(rejected),
-                "v{rejected} must be rejected by a v20 binary"
+                "v{rejected} must be rejected by a v21 binary"
             );
         }
     }

tests/bottom_panel_stage2b_protocol_acceptance.rs

@@ -218,7 +218,7 @@ fn appending_panel_events_does_not_move_the_previous_final_event_discriminant()
 
 #[test]
 fn a_panel_wider_than_512_columns_is_legal_while_a_terminal_is_not() {
-    let wide = MAX_TERMINAL_COLS as u32 + 1;
+    let wide = u32::from(MAX_TERMINAL_COLS) + 1;
 
     // The panel does not inherit the PTY per-axis cap: a 4K surface at a
     // small font is legitimately this wide, and the area bound is what
@@ -231,7 +231,7 @@ fn a_panel_wider_than_512_columns_is_legal_while_a_terminal_is_not() {
     assert!(matches!(
         terminal.validate(),
         Err(TerminalFrameError::Size { cols, max_cols, .. })
-            if cols == wide && max_cols == MAX_TERMINAL_COLS as u32
+            if cols == wide && max_cols == u32::from(MAX_TERMINAL_COLS)
     ));
 }