Flags must not overlap ledger lines or staff lines illegibly

A flag on a stem whose far end lands near or among ledger lines can visually merge with those lines. Engraving practice compensates via the stem-length extension for far-out notes (see `stem-length-extension-for-ledger-line-notes`) so the flag has clear space.

A sixteenth/thirty-second note several ledger lines from the staff whose flag visually overlaps a ledger line or the staff, because the stem's extension was not applied. Verified compounding cause: the SAME `getStemExtension()` bail-out described under `stem-length-extension-for-ledger-line-notes` (triggers whenever the manually-passed `stem_direction` disagrees with `calculateOptimalStemDirection()`) means flagged, ledger-line notes hit by the treble-pivot bug are doubly exposed — wrong direction AND missing the extension that would have kept the flag clear.

Ross, The Art of Music Engraving (flag-clearance is a stated reason for lengthening stems on far-out notes); Gould implies the same without naming flag-clearance as a separate rule.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.