So what mechanism calls a halt to “micro-evolution” and prevents it from becoming “macro-evolution”?
It would be the organism's inability to survive. If genetic change push is too far the organism can not survive. Most genetic mutations are fatal.
For example: An iguana can evolve to be able swim in salt water ( that would be micro evolution) but it would be unlikely to evolve to be a bird. At some point it would neither be well adapted for igunahood or for birdhood. :-)
But...Hey!...You are really pushing the envelop of what I learned in the 20 minutes we spent studying the topic on the undergrad level. :-)
When my daughter was 14 she took a biology major's course and ( because of her young age and inadequate background) she and I read every assigned page of her college text together. In a college text that was easily 2 to 3 inches thick, the author devoted 4 or 5 pages to macro-evolution.
Even in biology macro-evolution is really a tiny part of the study of biology.
Reality.