Notation you can trust (Epic M); release 0.5.0

[matteospanio]

Notation you can trust: Epic M, released as 0.5.0. A review found that the notation the note boards didn't measure was broken somewhere. This PR measures it (lyrics, chord symbols, dynamics, clefs, stems and beams, LilyPond's own playback) and fixes what the measurements showed. See docs/changelog.md for the list of changes, docs/devlog.md for the notes and docs/import-export.md for the measured support.

What changes

• MusicXML: read where <backup> and <forward> place the notes, and written valid (<divisions> always, text escaped, ISO-8859-1 input read).
• LilyPond reader: no longer drops constructs silently (\afterGrace, grace chords, tremolos, \partial N*M, beat structure, text scripts, placement, marks).
• Lyrics:
  • read and written the way LilyPond sings them (melismas, __, ~, punctuation, verses, named voices);
  • MusicXML elisions and verse names, MIDI syllables, ABC verses and Humdrum **text;
  • Note.lyric_syllables in Python.
• Chord symbols: kept as steps, so 7#9, add9 or m(maj7) are no longer read as major. They are written to LilyPond as g4:7.9- at their beat.
• Stems and beams: readers keep only what the source states, and writers draw the rest by LilyPond's rules. ABC and kern now have beams, and retrograde flips them.
• Note arrays: velocity per part, dynamics from directions, sounding pitch, grace chords.
• Writers:
  • church modes keep their tonic;
  • tempo is written once, and marks sit at their beat;
  • mid-bar clefs keep their place in every format;
  • MIDI never puts two instruments on one channel;
  • kern rests a voice that is absent from a bar.
• Transforms: retrograde carries key, meter and clef changes; transposition prefers fewer accidentals.
• CLI: every movement is written; LilyPond warnings go to stderr; -I works on every reading command; diff compares timing.

The last commit fixes what failed CI on the release commit:

• Clippy 1.99's new lints.
• The LilyPond oracle: LilyPond 2.24 writes a lyric tie as ‿ in its MIDI, and 2.22 writes ~. The oracle now compares them as equal.
• The fuzz job's crash: the beam rules listed every beat of the bar. A meter of 2³¹ beats used more than 8 GB in every writer and took down the runner. Equal beats are now kept as one.

Measured

Check	0.4.0	0.5.0
Lyrics surviving MusicXML → LilyPond / kern / MIDI (of 38 files)	8 / 0 / 17	33 / 38 / 33
Chord symbols through LilyPond (of 8)	0	7
LilyPond lyric cases where lytk reads what LilyPond sings	11 of 26	31 of 31
Syllables LilyPond sings where lytk wrote them	not measured	710 of 710
Stems / beams matching the source engraver	not measured	98.0 % / 94.1 %

Checks

• 1,206 Rust and 325 Python tests pass locally, and Clippy 1.99 is clean on every target.
• LilyPond oracle with both LilyPond 2.22.1 and 2.24.3: 31/31 lyric cases, 710/710 syllables, 148 fixtures compiled.
• The LilyPond corpus boards pass.
• The LilyPond-shaped fuzz (release build, 1,024 cases) passed three runs, peaking at 0.5 GB. Before the fix it hit an 8 GB cap within seconds.

🤖 Generated with Claude Code