Four kinds of malformed MusicXML are still absorbed in silence

[alessonqueirozdev-hub]

Problem

parseMusicXML / scoreFromMusicXML accept an optional warnings list, so a partial import is no longer indistinguishable from a complete one. Coverage is not total, and this issue tracks the residue rather than letting the README carry it alone.

Re-probed for 2.8.0 over nine malformed documents:

Document	Behaviour
<pitch> with no <octave>	rejected — FormatException
unknown <step>	rejected — FormatException
not a score at all	rejected — FormatException
<divisions>0</divisions>	warned
non-positive <duration>	warned
unknown <clef><sign>	SILENT
unknown <type>	SILENT
non-numeric <alter>	SILENT
missing <part-list>	SILENT

Why the silent four matter

Each one changes what the score means without saying so. An unknown <clef><sign> silently keeps the previous clef, so every pitch after it is engraved on the wrong line. A non-numeric <alter> drops an accidental. An unknown <type> drops a duration. A missing <part-list> loses staff naming and grouping.

Acceptance criteria

• Each of the four raises a warning naming the element and the offending value.
• The decision between warn and reject is stated per case, not chosen ad hoc — the current rule is that a defect which makes the surrounding music unreadable rejects, and one that loses a detail warns.
• A test per case, in the same shape as the existing malformed-document probes.