Part of #97. Depends on B7b/B7c.
Choose the prefix base per fragment: use the real prior trail when it's high-quality (enough points, length ≥ ~0.75*keyWidth, endpoints near the prefix's first/last keys), else the ideal key-center trail. Tap prefixes always ideal.
Acceptance: golden-corpus (#21) A/B — hybrid ≥ max(raw, ideal) across the §5 combo matrix; ship the winning default, keep the others behind a debug pref.
Part of #97. Depends on B7b/B7c.
Choose the prefix base per fragment: use the real prior trail when it's high-quality (enough points, length ≥ ~0.75*keyWidth, endpoints near the prefix's first/last keys), else the ideal key-center trail. Tap prefixes always ideal.
Acceptance: golden-corpus (#21) A/B — hybrid ≥ max(raw, ideal) across the §5 combo matrix; ship the winning default, keep the others behind a debug pref.