Consolidate scoped to discriminator set (no cross-project merges)

[CheeryProgrammer]

Part of #113 (Layer 1). Depends on scoped identity.

ProposeMerges/ApplyMerge must only consider candidates within the same
discriminator set. Two same-named nodes in different projects are never proposed
for merge; global merges only with global.

Acceptance: Consolidate never proposes merging OrderService{project:A} with
OrderService{project:B}; within one project, near-duplicate names still merge as today.

[CheeryProgrammer]

Shipped in PR #123, released v1.12.0. ProposeNodeMerges groups by (scope_key, normalized_name); cross-project same-name never proposed.