Enforce cross-project URI uniqueness for generated identifiers

[jaybea]

Context

• The current identifier strategy in Epic: Concept Identifier Strategy #133 defines uniqueness per project, across all schemes.
• It does not define whether generated concept URIs must also be unique across projects.
• If two projects share the same namespace and identifier prefix, they can independently mint the same identifiers and therefore the same URIs.

Question

Should we enforce cross-project URI uniqueness?

If yes, at what level?

• (namespace, identifier_prefix) uniqueness
• Partial uniqueness for non-null namespaces, with separate handling for null namespace
• Service-layer validation
• URI-level enforcement

Notes

• This is adjacent to Epic: Concept Identifier Strategy #133 but not required to complete the current project-scoped identifier work.
• It needs an explicit product/architecture decision before implementation.