[patch] Keep Tempo.TryParse from throwing, and reject infinite tempos and non-positive beats

[matt-edmondson]

Fixes #305

What was wrong

Tempo had two validation gaps:

• TryParse could throw. Its guard was bpm <= 0.0, which is false for NaN, so "NaNbpm@1/4" reached Create, and Create threw ArgumentOutOfRangeException. That breaks the Try-pattern contract for untrusted text.
• Values that make Seconds meaningless were accepted. An infinite tempo, and a zero or negative beat (0/4, -1/4), passed both TryParse and Create. Seconds divides by the beat, so these gave Infinity or negative time.

Change (Semantics.Music/Tempo.cs)

• Two private predicates hold the rules: IsValidBeatsPerMinute (positive and finite) and IsValidBeat (Numerator > 0; Duration normalises the sign onto the numerator, so 1/-4 is caught too).
• Create throws ArgumentOutOfRangeException for a non-finite or non-positive tempo, and for a non-positive beat (after the existing null check).
• TryParse applies the same predicates itself, so it returns false for these inputs and never reaches a throwing Create.
• Updated the XML docs to match.

The triage also mentioned Duration.TryParse accepting 0/4 and -1/4. I left Duration alone because zero and negative durations may be legitimate for other consumers (for example, differences). This PR only stops Tempo from using one as its beat.

Tests (TempoTests)

• TryParseReturnsFalseForInvalidTempo: NaN, -NaN, ±Infinity, 0, and -120 bpm, plus 0/4, -1/4 and 1/-4 beats. Each returns false with a null result.
• TryParseAcceptsValidTempo: "120bpm@1/4" still parses and round-trips.
• CreateThrowsForInvalidBeatsPerMinute: NaN, ±Infinity, 0 and -1.
• CreateThrowsForNonPositiveBeat: 0/1, 0/4, -1/4 and 1/-4.

With Tempo.cs reverted, 11 of these cases fail. With the fix, the full Semantics.Test suite passes: 1499 passed, 0 failed, 8 skipped. Semantics.Music builds for every target framework.

🤖 Generated with Claude Code

https://claude.ai/code/session_019RYN9jT6szUiySkywFKCA1

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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