a synthesized `score-instrument` id is process-global and leaves `midi-instrument` dangling

[webern]

When a part has MIDI data but an empty InstrumentData::uniqueId:

• midi-instrument/@id is written from core::Token{""}, which is X
  (src/private/mx/impl/PartWriter.cpp:188).
• score-instrument/@id is synthesized as ID1000000, ID1000001, ... from a process-wide atomic
  counter (PartWriter.cpp:64-68, PartWriter.cpp:260-275).

So midi-instrument/@id does not refer to the score-instrument. The output also depends on how
many scores the process has already written, so writing the same ScoreData twice gives different
files.

Related: #397. Found while auditing silent recoveries for #432 (#439).