Separate the union of timeslice and result in struct mrb_task

Bug scenario:
* VM returns an Exception `t->state.result = mrb_vm_exec(...);`
* Despite task is still MRB_TASK_STATUS_RUNNING, IRQ triggered by chance and `mrb_tick()` executes `t->state.timeslice--;`
* But the same memory area already holds the `result`, `timeslice--` reduces `result.value.p`'s top byte

The fix is to separate `timeslice` and `result` into different fields.
I have considered improving critical sections, but I ended up with this patch because I believe it is widely effective and less error-prone.

mrbgems/mruby-task/include/task.h

@@ -41,7 +41,6 @@ struct mrb_task_queue;
  * - Removed started flag (inferred from c.status): 1 byte
  * - Unified wakeup_tick/join/mutex into single union: 4 bytes
  * - Removed redundant proc field (stored in c.ci->proc): 8 bytes
- * - Unified timeslice/result into state union: ~4 bytes
  * Total savings: ~18 bytes per task (14% reduction)
  */
 typedef struct mrb_task {
@@ -61,11 +60,8 @@ typedef struct mrb_task {
 
   mrb_value self;                  /* Ruby Task object reference */
 
-  /* State-specific data - mutually exclusive based on status */
-  union {
-    volatile uint8_t timeslice;    /* Remaining ticks (RUNNING only) */
-    mrb_value result;              /* Task return value (DORMANT only) */
-  } state;
+  volatile uint8_t timeslice;      /* Remaining ticks while RUNNING */
+  mrb_value result;                /* Task return value */
 
   struct mrb_context c;            /* Execution context (stack, callinfo, etc) */
 } mrb_task;

mrbgems/mruby-task/src/task.c

@@ -114,7 +114,7 @@ mrb_task_mark_all(mrb_state *mrb)
       /* Mark task-specific values */
       mrb_gc_mark_value(mrb, t->self);
       if (t->status == MRB_TASK_STATUS_DORMANT) {
-        mrb_gc_mark_value(mrb, t->state.result);
+        mrb_gc_mark_value(mrb, t->result);
       }
       mrb_gc_mark_value(mrb, t->name);
 
@@ -326,7 +326,7 @@ execute_task(mrb_state *mrb, mrb_task *t)
 
   /* Set task as running */
   t->status = MRB_TASK_STATUS_RUNNING;
-  t->state.timeslice = MRB_TIMESLICE_TICK_COUNT;
+  t->timeslice = MRB_TIMESLICE_TICK_COUNT;
 
   /* Switch to task context */
   prev_c = mrb->c;
@@ -351,7 +351,7 @@ execute_task(mrb_state *mrb, mrb_task *t)
   t->c.vmexec = TRUE;
 
   /* Execute task - PC is saved in ci->pc from previous run */
-  t->state.result = mrb_vm_exec(mrb, proc, pc);
+  t->result = mrb_vm_exec(mrb, proc, pc);
 
   /* Clear vmexec flag */
   t->c.vmexec = FALSE;
@@ -394,9 +394,9 @@ mrb_tick(mrb_state *mrb)
 
   /* Decrease timeslice for running task */
   t = q_ready_;
-  if (t && t->status == MRB_TASK_STATUS_RUNNING && t->state.timeslice > 0) {
-    t->state.timeslice--;
-    if (t->state.timeslice == 0) {
+  if (t && t->status == MRB_TASK_STATUS_RUNNING && t->timeslice > 0) {
+    t->timeslice--;
+    if (t->timeslice == 0) {
       switching_ = TRUE;  /* Trigger context switch */
     }
   }
@@ -1115,7 +1115,7 @@ mrb_task_join(mrb_state *mrb, mrb_value self)
 
   /* If task is already dormant, return immediately */
   if (t->status == MRB_TASK_STATUS_DORMANT) {
-    return t->state.result;
+    return t->result;
   }
 
   /* Wait for task to complete */
@@ -1130,7 +1130,7 @@ mrb_task_join(mrb_state *mrb, mrb_value self)
   /* Trigger context switch */
   switching_ = TRUE;
 
-  return t->state.result;
+  return t->result;
 }
 
 /*
@@ -1190,16 +1190,16 @@ mrb_execute_proc_synchronously(mrb_state *mrb, mrb_value proc_val, mrb_int argc,
   mrb->c = &t->c;
 
   while (t->c.status != MRB_TASK_STOPPED) {
-    t->state.result = mrb_vm_exec(mrb, mrb->c->ci->proc, mrb->c->ci->pc);
+    t->result = mrb_vm_exec(mrb, mrb->c->ci->proc, mrb->c->ci->pc);
   }
 
   /* If there's an unhandled exception after VM stops, save it as result */
   if (mrb->exc) {
-    t->state.result = mrb_obj_value(mrb->exc);
+    t->result = mrb_obj_value(mrb->exc);
   }
 
   /* 5. Get result and clean up */
-  mrb_value result = t->state.result;
+  mrb_value result = t->result;
   if (mrb_obj_ptr(result) == mrb->exc) {
     mrb->exc = NULL;  /* Clear exception */
   }
@@ -1437,7 +1437,7 @@ mrb_task_value(mrb_state *mrb, mrb_value task)
   mrb_task *t = (mrb_task*)mrb_data_check_get_ptr(mrb, task, &mrb_task_type);
   if (!t) return mrb_nil_value();
 
-  return t->state.result;
+  return t->result;
 }
 
 /*