style: Add clarifying comment on protocol version assertion

Address CodeRabbit nitpick: explain that the hardcoded version check
is intentional to catch accidental protocol bumps.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2witstudios

crates/pu-core/src/protocol/tests.rs

@@ -480,6 +480,7 @@ fn given_kill_request_without_exclude_should_default_empty() {
 
 #[test]
 fn given_protocol_version_should_be_current() {
+    // Intentional hardcoded check — update when the protocol version is bumped.
     assert_eq!(PROTOCOL_VERSION, 4);
 }