CI: Add debug output for functional test investigation

GNU functional tests (mutex, semaphore) are timing out at 30s without
producing any QEMU output. Add debug output showing the first 500
characters of QEMU output to help diagnose whether:

1. Tests produce no output (indicating execution/build issue)
2. Tests produce output but fail to print PASS criteria
3. Tests are simply too slow and need more optimization

The tests enter infinite loops after printing results (by design to prevent
kernel panic), so QEMU never exits naturally. Regular app tests accept
timeout as valid, but functional tests need specific PASS criteria strings.

Author: jserv

.ci/run-functional-tests.sh

@@ -58,6 +58,15 @@ test_functional_app() {
 	output=$(timeout ${TIMEOUT}s qemu-system-riscv32 -nographic -machine virt -bios none -kernel build/image.elf 2>&1)
 	exit_code=$?
 
+	# Debug: Show first 500 chars of output
+	if [ -n "$output" ]; then
+		echo "[DEBUG] Output preview (first 500 chars):"
+		echo "$output" | head -c 500
+		echo ""
+	else
+		echo "[DEBUG] No output captured from QEMU"
+	fi
+
 	# Parse expected criteria
 	local expected_passes="${FUNCTIONAL_TESTS[$test]}"
 	IFS=',' read -ra PASS_CRITERIA <<<"$expected_passes"