test(protocol): pin that a ShellSpec from an older peer still decodes

Every wire struct here defaults its later fields so an older peer's message
degrades instead of failing the frame, and the rule is guarded — removing
`#[serde(default)]` from `PaneInfo` or `DaemonVersion` fails a test. Removing
either of `ShellSpec`'s passed the whole suite.

It is not a struct that can afford it. A `ShellSpec` crosses the wire inside
every `Spawn` that names a shell, in both directions and between builds: a
GUI talks to whatever `tty7-server` is installed on the far machine, and that
is regularly older. A required field there does not degrade — the frame fails
and the pane never starts.

`default_spawn_stays_wire_compatible_with_old_daemons` looked like it covered
this and does not: it exercises the `shell: None` legacy encoding, so it never
constructs one. This decodes the JSON an older peer sends and round-trips a
spawn that names a shell. Both defaults are checked by removal.

crates/tty7-core/src/daemon/protocol.rs

@@ -2322,6 +2322,49 @@ mod tests {
         assert_eq!(info.title, "");
     }
 
+    /// A `ShellSpec` from a peer that predates its later fields still decodes.
+    ///
+    /// This crosses the wire inside every `Spawn` that names a shell, in both
+    /// directions and between builds — a GUI talks to whatever `tty7-server`
+    /// is installed on the far machine, and that is regularly older. A field
+    /// without `#[serde(default)]` here does not degrade, it fails the frame,
+    /// and the pane never starts.
+    ///
+    /// `default_spawn_stays_wire_compatible_with_old_daemons` covers the
+    /// `shell: None` encoding and so never constructs one of these; removing
+    /// either default below passed the whole suite before this existed.
+    #[test]
+    fn a_shell_spec_from_an_older_peer_still_decodes() {
+        let spec: ShellSpec = serde_json::from_str(r#"{"program": "/bin/zsh"}"#).unwrap();
+        assert_eq!(spec.program, "/bin/zsh");
+        assert!(
+            spec.args.is_empty(),
+            "args must default, not fail the frame"
+        );
+        assert!(
+            !spec.args_are_tty7_defaults,
+            "a peer that never heard of this flag is not claiming tty7 defaults"
+        );
+
+        // And the same message inside the frame it actually travels in.
+        let msg = ClientMsg::Spawn {
+            cwd: None,
+            size: SIZE,
+            shell: Some(spec),
+            owner: None,
+            workspace: None,
+            restore: None,
+        };
+        let mut buf = Vec::new();
+        msg.encode(&mut buf).unwrap();
+        let (kind, payload) = read_frame(&mut std::io::Cursor::new(&buf)).unwrap();
+        assert_eq!(
+            ClientMsg::from_frame(kind, payload).unwrap(),
+            msg,
+            "a spawn naming a shell round-trips"
+        );
+    }
+
     /// The reason `previously_known_as` is a field and not a variant: this
     /// prompt is decoded by whatever GUI or `tty7-server` is at the other end,
     /// and one of them is regularly older than the build that sent it. A