The scope-prefix concatenation cacheKey used a bare U+FFFD separator with
no escaping, so an adversarial subject/tenant id containing that character
could collide with a different subject/tenant pair and leak cached roles
across tenants. Switch to JSON.stringify([scopeKey, subjectId]) for an
unambiguous key.
Also replace the untested key.endsWith() substring sweep used to
invalidate a subject across all tenants on a global write with an
explicit bySubject index, and add test coverage for both the collision
and the cross-tenant invalidation sweep.
Clintchiz