Split consolidation promotion payload helpers

[yvette-carlisle]

Summary

• Split consolidation promotion payload/ref/writegate helpers into consolidation/promotion/payload.rs.
• Kept DB transaction orchestration and SQL row updates in consolidation/promotion.rs.
• Added focused payload helper tests, including writegate wrapper rejection mapping.

Validation

• cargo make fmt-rust && cargo make fmt-rust-check && cargo make check-rust && cargo make lint-vstyle && git diff --check
• cargo nextest run -p elf-service consolidation::promotion::payload::tests::validate_promoted_memory_payload_maps_writegate_rejections --all-features
• cargo make lint-rust && cargo make test-rust && cargo make check (375 passed, 92 skipped)
• cargo make test-rust-integration with Docker Postgres/Qdrant (92 passed, 1 leaky, 375 skipped; exit code 0)
• cargo make check-trace-gate

Review Gates

• Scout selected this slice as a bounded promotion payload/module-boundary split.
• Skeptic reviewed the final diff and validation evidence with no blockers.