Do not fail successful browser probes when browser close exceeds cleanup timeout

## Problem

`wordpress.editor-open` and `wordpress.editor-validate-blocks` can complete navigation, readiness, and artifact capture successfully, then report the whole recipe step as failed because browser cleanup exceeds the fixed 1000ms close timeout. A seven-surface fixture run recorded four `runtime_execution_failed` findings solely from `operation=browser-close-timeout`.

## Evidence

- Homeboy run: `b280bf88-2257-4915-9d80-f4c52ff6a538`
- WP Codebox: `0.18.4`
- Browser: HeadlessChrome `151.0.7922.34`
- Fixture: `37-art-gallery-exhibition`
- Affected steps: editor-open and editor-validate-blocks
- Captured summaries show navigation/readiness succeeded and artifacts were written before close cleanup timed out.

Reproduction:

```sh
node tools/run-fixture-matrix.mjs --static-site-importer /path/to/static-site-importer --blocks-engine /path/to/blocks-engine --local --target-fixture 37-art-gallery-exhibition --surface-coverage 6
```

## Acceptance

- Browser close cleanup has a realistic bounded timeout or terminates the browser process deterministically.
- A cleanup timeout retains diagnostics without converting an otherwise successful probe and its valid artifacts into a workload failure.
- Repeated multi-surface editor-open and editor-validate-blocks runs complete without close-timeout findings.

## AI Assistance

OpenAI GPT-5.6 Sol via OpenCode ran the seven-surface matrix, inspected retained editor artifacts, and drafted this report. Chris Huber reviewed the evidence and remains responsible for the issue.

Run tools/run-fixture-matrix.mjs with the target fixture and surface coverage from the reproduction, then trace cleanup for the wordpress.editor-open and wordpress.editor-validate-blocks probes. Done means cleanup remains bounded or deterministic, diagnostics are retained, successful probes do not become failures, and repeated multi-surface runs have no browser-close-timeout findings.

Written by the indexing model from the issue text.