refactor: rename internal 'project' terminology to 'benchmark'

[christso]

Background

In #1019, the studio API endpoints were renamed from /api/projects to /api/benchmarks to align with user-facing labels. However, the internal TypeScript code still uses "project" terminology throughout.

Scope

Rename internal identifiers from "project" → "benchmark":

• ProjectEntry type → BenchmarkEntry
• registry.projects → registry.benchmarks
• getProject() / addProject() / removeProject() / discoverProjects() → getBenchmark() etc.
• withProject() route helper → withBenchmark()
• Route param :projectId → :benchmarkId
• projectId variables in handlers → benchmarkId
• Related test fixtures and helper names

Goal

Consistent terminology between the public API surface and internal implementation.

Non-goals

• No behavior changes
• No schema or wire format changes