Per-update policy for fanning a global profile change out to each of
the user’s joined rooms as a fresh m.room.member event. Mirrors the
MSC4466 propagate_to axis.
Validate a profile field name against the Common Namespaced Identifier
Grammar: a lowercase-leading identifier over [a-z0-9_.-], matching the
reference homeserver. Length is bounded separately by MAX_KEY_LENGTH.