docs(aria-snapshot): clarify coordinate system for boxes option

[pavelfeldman]

Summary

• Document that bounding boxes emitted by ariaSnapshot({ boxes: true }) are viewport-relative, in CSS pixels (i.e. Element.getBoundingClientRect()).
• Apply the same clarification to the MCP browser_snapshot tool and CLI snapshot command descriptions.

[github-actions]

Test results for "MCP"

3 failed
❌ [webkit] › mcp/dashboard.spec.ts:186 › should start dashboard and annotate when no dashboard is running @mcp-windows-latest-webkit
❌ [webkit] › mcp/dashboard.spec.ts:302 › should cancel browser_annotate when the MCP request is aborted @mcp-windows-latest-webkit
❌ [webkit] › mcp/dashboard.spec.ts:333 › should cancel browser_annotate when the MCP client disconnects @mcp-windows-latest-webkit

6886 passed, 1015 skipped

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Merge workflow run.

[github-actions]

Test results for "tests 1"

1 failed
❌ [firefox-library] › library/har.spec.ts:976 › tracing.startHar › should write HAR live while recording @firefox-ubuntu-22.04-node20

4 flaky

⚠️ [chromium-library] › library/video.spec.ts:682 › screencast › should capture full viewport on hidpi `@ubuntu-22.04-chromium-tip-of-tree`
⚠️ [chromium-library] › library/browsertype-connect.spec.ts:683 › run-server › should record trace with sources `@chromium-ubuntu-22.04-node20`
⚠️ [webkit-page] › page/page-set-input-files.spec.ts:38 › should upload a folder `@webkit-ubuntu-22.04-node20`
⚠️ [playwright-test] › ui-mode-trace.spec.ts:812 › should update state on subsequent run `@windows-latest-node20`

41590 passed, 847 skipped

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Merge workflow run.