feat(admin-ui): A/B experiments console card + Phase-5 close-out (Step 5.7d)

[officialCodeWork]

Summary

Step 5.7d — the final slice of Step 5.7 (A/B testing & shadow mode) and the Phase-5 close-out.

• A/B experiments card on the admin Live Status page (apps/admin-ui, /status) surfacing GET /v1/status/experiments — one row per running experiment with:
  • the lift (relative change of candidate vs control), colour-coded
  • a verdict badge: candidate wins (significant + positive), candidate worse (significant + negative), no change (analysed, not significant), or collecting (insufficient data)
  • the control→candidate means, the confidence interval on the difference, and per-arm sample counts
  • live-wired via useLive (fetchExperiments + seedExperiments) with a seed/demo fallback when no gateway is configured
• New ABAnalysis TS type mirrors rag_core.eval.ABAnalysisResult.
• No backend change — the /v1/status/experiments endpoint shipped in 5.7a.

Phase-5 close-out

Step 5.7 is now complete, so Phase 5 — Eval & Observability is done (7 / 7). The TRACKER milestone counts move to 64 / 84, and the next action points to Phase 6 (Step 6.1 — Logical multi-tenancy). Deferred items remain documented (per-tenant drift / per-dimension embedding PSI; feedback/breaker/quota Grafana export + Loki-events dashboard; gRPC proto mirror of corpus_decision + experiment; sequential / multi-metric experiments).

Documentation

• docs/reference/admin-ui.md — Live Status card list + prose now include the A/B experiments card
• docs/reference/experiments.md — new "Console (Step 5.7d)" subsection
• TRACKER.md — 5.7 + 5.7d ✅, Phase 5 complete, milestone counts, next action → Phase 6, PR history

Test plan

• admin-ui tsc --noEmit clean
• admin-ui eslint clean (changed files)
• admin-ui 34 vitest pass — incl. a new spec asserting the card renders the significant win (router_v2, +24.0%, "candidate wins") and the collecting run (reranker_v3, "21 samples")
• admin-ui next build succeeds (/status prerenders)
• browser-verified on the dev server: the card renders both states with correct styling/colour; no card-related console errors (the pre-existing relTime() hydration warning on seed timestamps is unrelated)

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