Standard stem length is one octave (3.5 staff spaces)

A stem on a notehead within or near the staff is drawn to a length of about one octave (3.5 staff spaces) from the notehead, scaled to the staff, not a fixed pixel length.

A rendered stem whose length (in staff-spaces, using the rendered staff's own line spacing) deviates from 3.5 for a notehead with zero ledger lines and no beam/flag extension, inconsistent across notes at the same nominal duration.

Gould, Behind Bars ('stems... are normally the length of an octave'); Ross, The Art of Music Engraving (3 1/2 spaces). Verified directly in VexFlow source rather than assumed: `Tables.STEM_HEIGHT = 35` and `Tables.STAVE_LINE_DISTANCE = 10` (tables.ts), giving 35/10 = 3.5 staff-spaces exactly as the base stem length before any extension — Gould/Ross's figure is a literal constant in the renderer, not an approximation.

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