Duration accepts zero and negative lengths, so Note, Rest, ChordEvent and Progression.Parse accept "C4:-1/4:v64", "R:0/4", "C@-1/4" #353

What's wrong

• Duration.Create (Semantics.Music/Duration.cs:42-55) rejects only a zero denominator.
• Duration.TryParse explicitly allows a leading sign (NumberStyles.AllowLeadingSign, :104-105).
• The following all accept whatever Duration holds:
  • Note.Create (Note.cs:27)
  • Rest.Create (Rest.cs:19)
  • ChordEvent.Create (ChordEvent.cs:23)
  • their TryParse counterparts
  • the Chord@n/d cell in Progression.Parse (Progression.Parse.cs:149)

So musical events with zero or negative length get through at every entry point.

Reproduction (net10.0)

Note.Seconds(tempo) and Rest.Seconds(tempo) (Note.cs:39-42, Rest.cs:29-32) then return negative or zero time.

Why it matters

Negative events silently subtract from Progression.TotalDuration and TotalBars, and from anything built on them, such as the bar logic in Form and Section. Related: #305 fixes the same hole, but only for Tempo's beat unit.

Suggested fix / acceptance criteria

• Pick one of these:
  • Make Duration strictly positive: reject numerator <= 0 in Create, and return false from TryParse.
  • Keep Duration as a general signed rational, and enforce > 0 in Note.Create, Rest.Create, ChordEvent.Create and Progression.Parse.
• All five inputs above are rejected: TryParse returns false and Create throws ArgumentOutOfRangeException.
• The zero Duration that seeds the TotalDuration aggregate still works, for example as an internal zero.