Memory leaks in instrument ownership model

Multiple memory leaks and crashes exist in instrument management:

Memory leaks: Part destructor missing, instruments never freed
Double-free risk: InstrumentChange deletes shared pointers
Use-after-free: setInstruments({}) after shallow-copy leaves dangling pointers
EID assertion: No idempotent registration check causes crashes on memory reuse
Thread-safety: Parallel excerpt serialization races on MIDI mapping

Root cause:

No ownership model for Instrument pointers shared between InstrumentChange and Part::m_instruments.

Checklist

• This report follows the guidelines for reporting bugs and issues
• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible

The report names no files or tests. Start by locating Part, InstrumentChange, Part::m_instruments, and the excerpt serialization path, then trace pointer ownership and concurrent MIDI mapping access. Done requires resolving the listed leaks, double-free and use-after-free risks, registration assertion, and serialization race with regression coverage.