Mimalloc arena `mi_heap_malloc_aligned` has a hidden limit on max alignment of 64 KiB per allocation

Start with the mi_heap_malloc_aligned, mi_manage_os_memory_ex, and mi_heap_new_in_arena API documentation, then run the provided v3 C reproduction using a 128 KiB alignment. Document the observed 64 KiB per-allocation alignment limit clearly; larger or unlimited alignment support is a separate follow-up if pursued.

I understand that there may be an internal limit on the max alignment for mi_heap_malloc_aligned of 64 KiB. In the short-term, the documentation should have this limit documented clearly. In the long-term, I wonder if there is way that we could support a larger limit or with no limit?

Repro (version: v3):

#include <stdint.h>
#include <stdio.h>
#include <sys/mman.h>

#include "mimalloc.h"

int main(void) {
  const size_t arena_size = 1ULL * 1024 * 1024 * 1024;
  const size_t buffer_size = 1 * 1024 * 1024;
  const size_t alignment = 128 * 1024;

  void* arena = mmap(NULL, arena_size, PROT_READ | PROT_WRITE,
                     MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  if (arena == MAP_FAILED) {
    perror("unable to mmap the arena");
    return 1;
  }

  mi_arena_id_t arena_id = NULL;
  if (!mi_manage_os_memory_ex(arena, arena_size, true, false, true, -1, true, &arena_id)) {
    fprintf(stderr, "unable to register the mmap region as an exclusive arena\n");
    return 1;
  }

  mi_heap_t* heap = mi_heap_new_in_arena(arena_id);
  if (heap == NULL) {
    fprintf(stderr, "unable to create a heap in the exclusive arena\n");
    return 1;
  }

  void* buffer = mi_heap_malloc_aligned(heap, buffer_size, alignment);
  if (buffer == NULL) {
    fprintf(stderr, "unable to allocate a buffer with 128 KiB alignment from the arena\n");
    mi_heap_destroy(heap);
    return 1;
  }

  const bool is_aligned = ((uintptr_t)buffer % alignment) == 0;
  const bool is_in_arena = mi_arena_contains(arena_id, buffer);
  mi_free(buffer);
  mi_heap_destroy(heap);

  if (!is_aligned || !is_in_arena) {
    fprintf(stderr, "allocation is not 128 KiB-aligned inside the registered arena\n");
    return 1;
  }

  return 0;
}