model hammer-on and pull-off as paired spanners in `mx::api`

[webern]

What

<hammer-on> and <pull-off> are paired, note-attached spanners inside <technical>, carrying a
required type (start/stop), an optional number, and text content for the printed label:

<technical><hammer-on type="start" number="1">H</hammer-on></technical>

mx::api has no representation for either. MarkType has both members commented out
(src/include/mx/api/MarkData.h:142-143), as do the Converter::technicalMarkMap entries
(src/private/mx/impl/Converter.cpp:330-332). They are dropped on read and cannot be authored.
mx::core models them fully as core::HammerOnPullOff behind
core::TechnicalChoice::Kind::{hammerOn, pullOff}, so the data is available and thrown away in
mx::impl.

Requirements

1. Read and write start/stop attached to the notes that carry them.
2. Honor number via the existing SpannerNumber mechanism.
3. Preserve the element text (the H / P label).
4. Pin a corpus file in src/private/mxtest/api/roundtrip-baseline.txt, e.g.
   ksuite/k004a_Technical.xml. Both are support="none" in data/api.features.xml, and none of
   the five corpus files using them are defended today.

Follow whatever shape #389 settles on for the other note-attached paired spanners. tap (the third
member of audit section 1.2) is not in scope; it carries text only and is not a spanner.

Blocks u_span eligibility in #393. Related: #389, #185 (closed; deferred these).