feat: model pedal number so overlapping pedal lines keep their identity

[webern]

Human Summary

The ability to specify the line level for pedal lines was missing. Added here.

Summary

mx::api::PedalLineData now carries a SpannerNumber number, and both translation directions
honour MusicXML's <pedal number="..."> attribute. The field is the same identity type wedges,
curves and wavy lines already use: leave it unspecified for a lone pedal line, give an explicit
level to write that level verbatim, or give the events of one line a shared identity label and the
writer assigns a level from serialization order.

The reader takes the number off the element in DirectionReader::parsePedal. The writer emits it
from DirectionWriter::emitPedal through SpannerResolver, which now gives pedal lines their own
pool of numbers 1..16 like every other spanner family. A pedal line opens with start, sostenuto
or resume and closes with stop or discontinue; change and continueLine happen while the
line stays open.

Nothing changes for an ordinary score: a single pedal line with no number writes no number
attribute, and every other spanner family keeps its own pool and assignments.

Deliberately left out, as separate gaps: pedal/@sign and pedal/@abbreviated (the sign form is
modelled through MarkType::pedal / MarkType::damp), the font and color half of
print-style-align on <pedal>, and numbers on the pedal sign form.

Testing

• New reader test: <pedal type="start" line="yes" number="2"> reads as PedalLineKind::start with explicit level 2; a pedal with no number reads as unspecified
• New resolver/writer tests: two overlapping identity pedal lines get distinct numbers and both survive a round trip; an explicit pedal level is written verbatim and round-trips; a pedal number and a wedge number come from separate pools
• New round-trip tests: an explicit pedal number comes back; a pedal line without a number writes no number attribute
• Reverting just the reader/writer/resolver sites fails 5 of the new test cases (10 assertions), so they are real regressions
• make api-test: all pass (5909 assertions in 664 test cases)
• make api-roundtrip: 414 passed, 0 failed (of 414 pinned)
• make api-roundtrip-discover: 414 PASS, 426 FAIL, 0 LOADFAIL/GETDATAFAIL/CREATEFAIL -- no file unlocked, so no baseline pin. synthetic/pedal.3.1.xml still fails because its pedal also carries abbreviated, font and color attributes that mx::api does not model
• make test-all, make fmt-check

References

• Closes expose pedal/@number in mx::api::PedalLineData #411
• Related to feat: check ID uniqueness and ID references #455
• Related to Support spanners specified as a single object instead of start and stop pairs. #393