Add stack overflow detection for coroutines

This implements canary-based stack overflow detection for hart coroutine
to catch stack corruption before it leads to undefined behavior.

The implementation provides immediate detection of stack corruption
with minimal performance cost.

Author: jserv

coro.c

@@ -207,6 +207,7 @@ typedef struct {
     coro_context_t *context; /* Context buffer */
     void *stack_base;        /* Stack base address */
     size_t stack_size;       /* Stack size */
+    uint64_t canary;         /* Stack overflow detection canary */
 } coro_t;
 
 /* Global state */
@@ -222,6 +223,9 @@ static struct {
 /* Stack size for each hart coroutine (1MB - increased for complex execution) */
 #define CORO_STACK_SIZE (1024 * 1024)
 
+/* Stack canary value for overflow detection */
+#define STACK_CANARY_VALUE 0xDEADBEEFCAFEBABEULL
+
 /* Sentinel value for current_hart when no coroutine is running */
 #define CORO_HART_ID_IDLE UINT32_MAX
 
@@ -241,6 +245,19 @@ static inline void coro_clear_running_state(void)
     tls_running_coro = NULL;
 }
 
+/* Check for stack overflow by verifying the canary value */
+static inline void coro_check_stack(coro_t *co)
+{
+    if (co->canary != STACK_CANARY_VALUE) {
+        fprintf(stderr,
+                "FATAL: Stack overflow detected in coroutine! "
+                "Expected canary=0x%llx, got=0x%llx\n",
+                (unsigned long long) STACK_CANARY_VALUE,
+                (unsigned long long) co->canary);
+        abort();
+    }
+}
+
 /* Forward declarations */
 
 #ifdef CORO_USE_ASM
@@ -483,6 +500,9 @@ bool coro_create_hart(uint32_t hart_id, void (*func)(void *), void *hart)
         return false;
     }
 
+    /* Initialize stack canary for overflow detection */
+    co->canary = STACK_CANARY_VALUE;
+
     /* Initialize context */
 #ifdef CORO_USE_ASM
     make_context(co, &co->context->ctx, co->stack_base, co->stack_size);
@@ -520,9 +540,15 @@ void coro_resume_hart(uint32_t hart_id)
         return;
     }
 
+    /* Check for stack overflow before resuming */
+    coro_check_stack(co);
+
     coro_state.current_hart = hart_id;
     co->state = CORO_STATE_RUNNING;
     jump_into(co);
+
+    /* Check for stack overflow after returning from coroutine */
+    coro_check_stack(co);
 }
 
 void coro_yield(void)