bench: visual verification of Studio comparison matrix UI

[christso]

Objective

Visually verify the Studio comparison matrix UI (PR #984) renders correctly in a browser. The API has been verified but the React component has not been tested in an actual browser.

Context

PR #984 adds a "Compare" tab to the Studio with an experiment × target matrix. The backend API returns correct data (verified via curl), but the React component (CompareTab.tsx) has not been tested in a browser because the dev environment lacks a display server for headless Chrome.

What to verify

1. Start studio: agentv studio --multi
2. Register agentv-bench-skills project (has experiment data)
3. Navigate to the project → "Compare" tab
4. Verify:
   • Matrix renders with experiments as columns, targets as rows
   • Cells are color-coded: green (>80%), yellow (50-80%), red (<50%)
   • Best/worst performer indicators (▲/▼) appear
   • Clicking a cell expands to show per-test-case breakdown
   • Empty state renders correctly for projects with no experiments
   • Error state renders when API fails
   • Loading skeleton shows while data loads

Acceptance criteria

• Screenshot evidence of the comparison matrix with real data
• Expand/collapse works
• Color coding is correct
• Responsive on different viewport sizes

[christso]

Verified in #995. Screenshot evidence, expand/collapse, and color coding all confirmed. Studio sidebar non-responsive on mobile is pre-existing (tracked separately in #996).