feat(studio): add targets tab drilldown

[christso]

Closes #1188

Summary

• make Targets rows clickable in Studio
• drill into experiment-grouped runs for the selected target
• reuse existing run detail pages for category breakdowns and individual test cases
• use the same TargetsTab drill-down for benchmark-scoped pages

Red/green UAT

Red

Baseline origin/main targets tab snapshot on the shared fixture showed only static cells:

• claude and gpt-4o rendered as plain table cells
• no interactive control existed to drill into runs by experiment

Green

Feature branch targets tab snapshot on the same fixture showed:

• claude and gpt-4o rendered as buttons in the Targets table
• clicking gpt-4o opened a drill-down with exp-b and exp-a sections, each with its run list
• clicking gpt-4o · exp-b opened the existing run detail page with Category Breakdown and All Evals visible

Verification

• bun run lint
• bun run build
• bun run test

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