I'm not familiar with any case law giving a definition of "stock proof".
It's also important to note that although straying animals do constitute a trespass to the land the courts in Scotland have been very reticent to award interdict to prevent repeated trespass. The keeper is expected to take "reasonable steps" to prevent trespass BUT the interpretation of that may surprise some. It is considered reasonable to confine chickens but not sheep and cattle and the courts have historically been unwilling to impose burdens of fencing on the keepers. Admittedly much of the case law is pretty old but it has not been considered in recent cases. There's a rather lovely case from 1885 where the judge said that the burden of fencing was a matter for the owner of the land trespassed upon
However, under statute, once an animal has strayed it can be detained by the owner of the land and is then treated as any other lost or abandoned property so must be reported to either the owner or the police. The police then have a discretion to require payment of a reward to the detentor of the animal. There is also strict liability for any damage caused by the animal where the land or produce of the land is damaged.