Model FSEvents stream-level conditions.

StreamCondition names the eight flags in the low byte of
FSEventStreamEventFlags, which were previously unnamed and silently
indistinguishable from 'nothing changed'. Most consequential:
mustScanSubDirectories, surfaced as FolderContentChangeEvent.requiresRescan,
which reports that FSEvents dropped events and the tree must be re-listed.

Change(eventFlags:) now masks those bits off, so an empty Change means the
element reports no file change. Change also gains the two item flags it was
missing: cloned and ownEvent.

Additive: no existing member's raw value moves, and the bit ranges are
disjoint.