Slice: Define pairwise DID authority-domain invariant

[flyingrobots]

Slice

Turn the pairwise DID invariant into a Continuum authority-domain requirement.

Source detail:

• participants must not use one global public key across contexts;
• Echo-facing activity and another participant/runtime's activity may use different pairwise DIDs;
• participants should not gain unnecessary correlation handles.

Done when:

• AuthorityDomainRef or equivalent is specified for profile evidence;
• pairwise DID scope and non-correlation requirements are explicit;
• descriptor-only and verified identity postures are distinguishable;
• participants can cite authority-domain refs without requiring identity verification in early evidence witnesses.

Parent: #58.