vm.c: use 1.5x stack growth instead of linear

Change default stack growth from linear (+128) to exponential (1.5x).
This reduces reallocation frequency while maintaining reasonable memory
usage. The minimum growth is still MRB_STACK_GROWTH (128) to ensure
small programs don't over-allocate.

MRB_STACK_EXTEND_DOUBLING (2x growth) remains available for maximum
performance when memory is not a concern.

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

doc/guides/mrbconf.md

@@ -61,12 +61,12 @@ end
 `MRB_STACK_EXTEND_DOUBLING`
 
 - If defined doubles the stack size when extending it.
-- Otherwise extends stack with `MRB_STACK_GROWTH`.
+- Otherwise extends stack with 1.5x growth (minimum `MRB_STACK_GROWTH`).
 
 `MRB_STACK_GROWTH`
 
 - Default value is `128`.
-- Used in stack extending.
+- Minimum stack growth size when extending.
 - Ignored when `MRB_STACK_EXTEND_DOUBLING` is defined.
 
 `MRB_STACK_MAX`

src/vm.c

@@ -169,13 +169,17 @@ stack_extend_alloc(mrb_state *mrb, mrb_int room)
   else
     size += room;
 #else
-  /* Use linear stack growth.
+  /* Use 1.5x stack growth.
      It is slightly slower than doubling the stack space,
      but it saves memory on small devices. */
-  if (room <= MRB_STACK_GROWTH)
-    size += MRB_STACK_GROWTH;
-  else
-    size += room;
+  {
+    size_t newsize = size + (size >> 1); /* 1.5x growth */
+    if (newsize < size + MRB_STACK_GROWTH)
+      newsize = size + MRB_STACK_GROWTH;
+    if (newsize < size + (size_t)room)
+      newsize = size + room;
+    size = newsize;
+  }
 #endif
 
   mrb_value *newstack = (mrb_value*)mrb_realloc(mrb, mrb->c->stbase, sizeof(mrb_value) * size);