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.

Start in browser-preview-routing.js around fetchBrowserPreviewRoutedHost and fulfillBrowserPreviewRoutedHost, using the ECONNRESET evidence from Homeboy run c5bd79ee-f9c7-479e-bb1f-73cca37e98d2 as context. Add regression coverage that injects an ECONNRESET for a subresource, then verify asset failures do not terminate the process while document navigation failures remain explicit scenario failures.

Written by the indexing model from the issue text.