XY-800: [ELF vNext] Reviewable consolidation proposal store and job contract

[yvette-carlisle]

Summary

• Add the elf.consolidation/v1 spec for review-gated derived proposals over immutable source refs.
• Add domain validation, SQL tables, storage queries, and fixture-driven service create/list/get/review flows for consolidation runs and proposals.
• Add deterministic tests for source refs, snapshot guards, lifecycle transitions, and destructive diff rejection.

Verification

• cargo make fmt
• cargo make lint-fix
• cargo make checks