XY-27: Viewer: add retrieval observability panels on top of the read-only viewer

[yvette-carlisle]

Summary

• Add read-only admin viewer trace observability panels for provider/ranking metadata, retrieval funnel metrics, selected final results, replay candidates, relation context, stage summaries, and stage detail payloads.
• Persist a readback-only retrieval score in trace replay candidate snapshots so trace bundle consumers can compare retrieval/fusion score with rerank/final ranking terms.
• Add per-query ELF trace IDs to live baseline report output and link benchmark guidance to the viewer trace panel.

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make checks
• cargo test -p elf-api admin_viewer_is_admin_prefixed_and_read_only
• cargo test -p elf-service replay_ranking_policy_id_matches_ranking_policy_id
• cargo test -p elf-service replay_diversity_decisions_prefer_selected_entry_for_same_note
• Node parse check for apps/elf-api/static/viewer.html embedded script