Equidistant chord extremes default to stem down

When a chord's top and bottom notes are equally far from the middle line, the stem defaults down — the same default used for a single note sitting exactly on the middle line.

A symmetric chord (distances of 6 above and 6 below the pivot, say) rendered stem-up, or inconsistent run-to-run direction for the equidistant case. Under the CURRENT buggy manual code this branch does not even exist as a distinct case — equidistant chords are silently swallowed by the same top-pitch-only comparison as the parent bug, so today's 'tie-break' is really 'whatever the wrong pivot happens to produce for the top note alone.'

Gould, Behind Bars (equidistant chords take a downward stem, consistent with her on-the-line-is-down rule at). Ross gives no explicit tie-break; Read is silent. Recommend Gould's rule for Gradus — it is the only authority stating one explicitly, and it matches VexFlow's own tie-break (`decider === MIDDLE_LINE` is not `< MIDDLE_LINE`, so falls to the DOWN branch — verified in stavenote.ts:567).

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.