mruby-fiber,mruby-task: increase stack init size for 32-bit msvc

increase fiber_stack_init_size and task_stack_init_size from 16 to 64
to fix crashes on 32-bit msvc builds. git bisect identified commit
3246dd2 (which reduced sizes from 64 to 16) as causing the issue.
empirical testing shows 48 fails intermittently but 64 is stable on
32-bit msvc, likely due to different alignment or initialization
overhead on 32-bit platforms.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-fiber/src/fiber.c

@@ -10,7 +10,7 @@
 
 #define fiber_ptr(o) ((struct RFiber*)mrb_ptr(o))
 
-#define FIBER_STACK_INIT_SIZE 16
+#define FIBER_STACK_INIT_SIZE 64
 #define FIBER_CI_INIT_SIZE 4
 /* copied from vm.c */
 #define CINFO_RESUMED 3

mrbgems/mruby-task/include/task.h

@@ -87,7 +87,7 @@ typedef struct mrb_task {
 #define MRB_TIMESLICE_TICK_COUNT 3  /* Number of ticks per timeslice */
 #endif
 
-#define TASK_STACK_INIT_SIZE 16   /* Initial task stack size */
+#define TASK_STACK_INIT_SIZE 64   /* Initial task stack size */
 #define TASK_CI_INIT_SIZE 4       /* Initial task callinfo size */
 
 /*