feat(dashboard): compare providers side by side

[christso]

Summary

• Preserve multi-provider results in /api/compare instead of deduping same-run test cases by test_id alone.
• Add a Dashboard Provider/model comparison table with per-test provider columns, score/pass status, duration, answer preview, and raw answer/grading/transcript links.
• Update Dashboard docs to describe provider/model comparison through providers and provider labels, not a separate models field.

Validation

• bun test apps/dashboard/src/components/AnalyticsTab.test.tsx
• bun test apps/cli/test/commands/results/serve.test.ts --test-name-pattern "GET /api/compare"
• cd apps/dashboard && bun run build
• bun --filter agentv build
• Live dogfood: Promptfoo eval eval-O2u-2026-07-07T13:55:57; AgentV eval .agentv/results/2026-07-07T14-06-40-813Z; Dashboard UAT via agent-browser on rebuilt apps/dashboard/dist.

Evidence

Private evidence is in EntityProcess/agentv-private branch evidence/av-94ni-model-compare-dogfood at commit b066718.

Beads

• av-nlpr
• av-94ni
• av-v9gy
• Follow-up created: av-9ywp.1 for a future differing-output filter.

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