A tie's curve must be placed on the side opposite the note's stem

The tie arc curves away from the stem. The primary engraving rule is actually POSITIONAL (notes above the middle staff line curve down, below curve up), and 'opposite the stem' is the corollary that holds in the common case because stem direction is itself set by the same middle-line rule — the two rules diverge only where stem direction has been FORCED for a reason other than pitch (e.g. Gradus's own SATB shared-stave stemming).

Gould ('Ties should be placed on the notehead side away from the stem'); Read states the same clearance principle; Ross documents the specific above/below convention by notehead position relative to the middle staff line.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.