docs: add comment explaining why perf test scenarios omit expect() assertions

## Context

The performance test scenarios in `browser_tests/tests/performance.spec.ts` (zoom sweep, pan many nodes, many nodes idle, minimap idle) collect and record metrics but don't call `expect()`. This is intentional - regression detection is deferred to `scripts/perf-report.ts` which compares metrics against baselines and posts GitHub PR comments with delta analysis.

## Proposed Solution

Add a brief comment at the top of the `describe` block explaining this design so future readers understand why there are no inline assertions.

## Origin

Identified during PR #9910 review (thread on `performance.spec.ts:1`).

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-9961-docs-add-comment-explaining-why-perf-test-scenarios-omit-expect-assertions-3246d73d365081d6840bdda671c97f82) by [Unito](https://www.unito.io)

Start in browser_tests/tests/performance.spec.ts and review the performance scenarios and their surrounding describe block. Read scripts/perf-report.ts to confirm where metrics are compared against baselines, then add a brief explanatory comment and verify that the comment clearly documents why inline expect() assertions are omitted.

Written by the indexing model from the issue text.