Music notation feature support

Existing documentation was treated as a lead rather than proof. Statuses were assigned from parser, model, layout, rendering, converter, fixture, and test source at the recorded snapshot. Partial labels describe the remaining boundary as a subset, approximation, lossy mapping, required extension, fixed style, or semantic gap. Upstream MNX and MusicXML descriptor text remains maintained by the W3C community and is not duplicated here.

• Upstream data currently contains a rest-1024th format entry that is absent from the visible taxonomy, and MNX assigns the invalid level 0 to struct-staff-hide-specific. The updater reports these anomalies without treating them as Viritura coverage.
• Viritura's strongest MNX-pipeline coverage is conventional pitched notation, rhythms/rests, common signatures/barlines, voices, repeats, and explicit system/page layout. Advanced tablature remains entirely unsupported.
• Viritura's MusicXML/MXL importer supports core notation outright and additional concepts partially. Many partial rows require includeVendorExtensions, which is off by default.
• The importer rounds MusicXML decimal pitch alterations to integers, so microtonal pitches are corrupted rather than merely losing their display glyph.
• MusicXML 22ma/22mb octave shifts fall through to 8va/8vb, and unrecognized dynamic spellings fall back semantically to f.
• Standalone tempo directives such as rit., accel., and a tempo are discarded unless the same measure also yields a numeric tempo object.
• A first-part key signature is skipped whenever its initial attributes also contain transposition; this avoids treating a written key as concert pitch but currently loses the key instead of converting it.