refactor: extract shared PerfMeasurement interface across tsconfig projects

## Context

The `PerfMeasurement` interface is defined independently in two files that live in separate tsconfig projects:
- `browser_tests/fixtures/helpers/PerformanceHelper.ts`
- `scripts/perf-report.ts`

When new metrics are added (e.g. `totalBlockingTimeMs`, `frameDurationMs`), both copies must be updated manually.

## Proposed Solution

Extract the interface to a shared `.d.ts` file that both tsconfig projects can reference, eliminating the duplication.

## Origin

Identified during PR #9910 review (thread on `PerformanceHelper.ts:30`).

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-9960-refactor-extract-shared-PerfMeasurement-interface-across-tsconfig-projects-3246d73d365081a4b7ccf97554b17c6d) by [Unito](https://www.unito.io)