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.

Empieza en browser-preview-routing.js, alrededor de fetchBrowserPreviewRoutedHost y fulfillBrowserPreviewRoutedHost, utilizando como contexto la evidencia de ECONNRESET del run de Homeboy c5bd79ee-f9c7-479e-bb1f-73cca37e98d2. Añade cobertura de regresión que inyecte un ECONNRESET para un subrecurso y verifica después que los fallos de assets no terminen el proceso, mientras que los fallos de navegación del documento sigan siendo fallos explícitos del escenario.

Escrito por el modelo de indexación a partir del texto del issue.