Add missing test_checkpoints.rs module

The test_checkpoint! macro was being used in main.rs but the module
file was never committed, causing CI build failures.

This module provides checkpoint markers for fast, signal-driven testing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: ryanbreen

kernel/src/test_checkpoints.rs

@@ -0,0 +1,15 @@
+//! Test checkpoint infrastructure for fast, signal-driven testing
+//!
+//! Only compiled when cfg(feature = "testing") is enabled.
+
+/// Emit a test checkpoint marker to serial output
+/// Format: [CHECKPOINT:name]
+#[macro_export]
+macro_rules! test_checkpoint {
+    ($name:expr) => {
+        #[cfg(feature = "testing")]
+        {
+            log::info!("[CHECKPOINT:{}]", $name);
+        }
+    };
+}