model technical marks with payloads (fingering, pluck, bend, ...) in mx::api #185

fingering, pluck, hammer-on, pull-off, bend, and tap are deliberately commented out of
technicalMarkMap (Converter.cpp:266) because they carry payloads a flat MarkType cannot hold
(text; start/stop; bend-alter/pre-bend/release). They fall back to MarkType::unspecified and are
lost. Unlike the section-1b enum fixes, each needs a new api data type, so this is a feature, not a
one-line fix. fingering (wild=8) is the most common.

Part of #159. Source: docs/ai/api-feature-audit.md section 1c. Implement via the add-mx-api-feature skill.