feat: carry structured location and exception cause in api errors

[webern]

Human Summary

Introduces a mechanism for getting better error information out of mx::api. It won't be very helpful to humans, but an LLM can probably make some sense of the indices involved in an error. And xmlPath seems like it could be useful even to a human. Nothing pretty or fancy here, but information nonetheless.

Summary

internalError was a dead end: it swallowed the exception type, said nothing about where the problem was, and a TODO in Result.h said so. This gives ApiError two things it was missing: a place and a cause.

Location says where, in whichever world the error happened:

struct Location
{
    std::string xmlPath;  // /score-partwise/part[1]/measure[3]/note[2]
    int partIndex = -1;         // ScoreData coordinates; -1 = unknown
    int measureIndex = -1;
    int staffIndex = -1;
    int voiceIndex = -1;
    int tickTimePosition = -1;
    long long byteOffset = -1;  // raw XML syntax errors
};

cause (std::exception_ptr) keeps the actual exception that was caught, so a caller can rethrow it, inspect its type, or read its message; std::bad_alloc now maps to its own ResultCode::outOfMemory instead of hiding in internalError. The catch blocks in MusicXml.cpp capture std::current_exception() into cause.

Two sources of position are wired now, without threading any new context through the call stack:

• The WriteRefusal sites in NoteWriter were already holding a MeasureCursor (part, measure, staff, voice, tick) when they refused a note; the error now carries it. A ninth beam comes back as tooManyElements at part=0 measure=0 staff=0 voice=0 tick=0.
• XML syntax errors carry pugixml's byte offset. Parse/schema errors keep the element path they already had; the io errors say the file path in their message instead of pretending it was an XML path.

formatError(const ApiError&) renders an error the same way every time (mx: tooManyElements at part=0 measure=0: ...), for logs, tests, and anyone (or anything) reading test output.

The path field is replaced by location; aggregate initialization of ApiError changes shape, so this is breaking, but only for code that constructs errors directly. mx::core is untouched: the core mirror maps Error::path into location.xmlPath and stays lossless.

This is groundwork for #432: a warnings mechanism wants somewhere to say where and something to attach; this interface now has both.

Testing

• ResultTest (new): Location defaults, formatError output pinned for bare codes, XML paths, score coordinates, and byte offsets
• tooManyElementsCarriesThePlaceInTheScore: a 9-beam note refuses with the full cursor position, rendered by formatError
• outOfMemoryIsReportedNotThrown: a std::bad_alloc thrown mid-parse comes back as outOfMemory, rethrowable from cause
• internalErrorKeepsTheException: an unexpected exception comes back as internalError with the original type and message preserved in cause
• Full suite passes: 5525 assertions in 610 test cases
• make api-roundtrip regression: 414 passed, 0 failed
• make fmt-check passes

References

• Progresses Consider a different error or warning mechanism. #432
• Stacked on feat: replace DocumentManager with RAII MusicXml #435; merges after it