Problem

MetricsDropdown.tsx and CompareV2.tsx hardcode [0] and [1] array indices to address the two comparison panels. There is no runtime guard if selectedArtifacts.length < 2, so any future change to the panel count (or a bug in the parent) would cause a silent crash.

Affected locations:

• frontend/src/components/viewers/MetricsDropdown.tsx — 6 occurrences (selectedArtifacts[0], selectedArtifacts[1], selectedArtifactsForDisplay[0], selectedArtifactsForDisplay[1])
• frontend/src/pages/CompareV2.tsx — getVerifiedTwoPanelSelection indexes [0]/[1], and createSelectedArtifactArray(2) seeds the magic number in 3 places

Suggested fix

• Extract the panel count into a named constant (e.g. COMPARE_PANEL_COUNT = 2).
• Derive the dropdown layout from the array length rather than hardcoded indices where possible.
• Add a runtime assertion or early return if selectedArtifacts.length doesn't match the expected count.

Context

Spotted during review of #13256. Pre-existing issue, not introduced by that PR.