Debug: Add diagnostic output to mutex test

The mutex test hangs immediately after enabling preemptive scheduling,
with only 'S' printed. Add diagnostic markers at:
- Task entry points (task_a, task_b, monitor_task)
- Before enabling preemptive scheduling

This will help identify if:
1. Tasks never start executing (scheduler issue)
2. Tasks start but hang on first operation (semaphore/sync issue)
3. Preemptive scheduler never activates (app_main return issue)

Temporary diagnostic commit to identify root cause of complete hang.

Author: jserv

app/mutex.c

@@ -18,6 +18,7 @@ static int currently_in_critical_section = 0;
 /* Enhanced Task A */
 void task_a(void)
 {
+    printf("[DIAG] Task A entry point reached\n");
     printf("Task A (ID %d) starting...\n", mo_task_id());
 
     for (int i = 0; i < MAX_ITERATIONS; i++) {
@@ -70,6 +71,7 @@ void task_a(void)
 /* Enhanced Task B */
 void task_b(void)
 {
+    printf("[DIAG] Task B entry point reached\n");
     printf("Task B (ID %d) starting...\n", mo_task_id());
 
     for (int i = 0; i < MAX_ITERATIONS; i++) {
@@ -130,6 +132,7 @@ void task_b(void)
 /* Simple monitor task */
 void monitor_task(void)
 {
+    printf("[DIAG] Monitor entry point reached\n");
     printf("Monitor starting...\n");
 
     int cycles = 0;
@@ -220,5 +223,6 @@ int32_t app_main(void)
            (int) task_b_id, (int) monitor_id, (int) idle_id);
 
     printf("Starting test...\n");
+    printf("[DIAG] About to enable preemptive scheduling\n");
     return true; /* Enable preemptive scheduling */
 }