Browser preview route crashes runtime on transient ECONNRESET

## Problem

A transient asset proxy reset in browser preview routing escapes the route handler as an unhandled rejection and terminates the entire WP Codebox recipe process.

## Evidence

During a valid multisite browser scenario, the runtime crashed while fetching a non-critical theme font:

```
route.fetch: read ECONNRESET
GET /wp-content/themes/extrachill/assets/fonts/Lobster2.woff2
at fetchBrowserPreviewRoutedHost (.../browser-preview-routing.js:287:36)
at fulfillBrowserPreviewRoutedHost (.../browser-preview-routing.js:219:28)
```

The backend suite had completed and no changed browser assertion executed. Homeboy run: `c5bd79ee-f9c7-479e-bb1f-73cca37e98d2`.

## Expected

Preview routing handles transient upstream fetch failures without crashing the Node process. The individual request may fail/abort and should be recorded as network evidence, while the browser scenario and cleanup remain under normal lifecycle control.

## Acceptance criteria

1. `route.fetch()` rejection is caught inside preview routing.
2. A failed non-document asset request does not terminate the recipe process.
3. Document navigation failures remain explicit scenario failures.
4. Regression coverage injects an `ECONNRESET` for a subresource.

## AI assistance
- **AI assistance:** Yes
- **Tool:** OpenCode with GPT-5.6 Sol
- **Used for:** Capturing and classifying the disposable browser-runtime crash.

browser-preview-routing.js の fetchBrowserPreviewRoutedHost と fulfillBrowserPreviewRoutedHost の周辺から着手し、コンテキストとして Homeboy の run c5bd79ee-f9c7-479e-bb1f-73cca37e98d2 における ECONNRESET の証拠を使用します。サブリソースに ECONNRESET を注入する回帰テストを追加し、その後、アセットの失敗ではプロセスが終了せず、ドキュメントナビゲーションの失敗は引き続き明示的なシナリオ失敗となることを確認します。

索引モデルが issue の本文から書いたものです。