Ensure book identifiers are uppercase in USFM output

[Copilot]

Book identifiers in USFM markers should always be uppercase regardless of input case.

Changes

• BTTWriterLoader.cs: Apply ToUpperInvariant() to manifest.project.id when setting IDMarker.TextIdentifier and TOC3Marker.BookAbbreviation
• Tests: Added coverage for lowercase and mixed-case book identifier inputs

// Before
document.Insert(new IDMarker() { TextIdentifier = manifest.project.id });

// After  
document.Insert(new IDMarker() { TextIdentifier = manifest.project.id.ToUpperInvariant() });

Original prompt

I need book identifiers inserted into USFM to always be caps, add tests for that too

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