Add fetch streaming/progress utilities

[marcstraube]

Context

RequestInterceptor wraps Fetch but does not expose ReadableStream progress tracking for large downloads/uploads.

Goal

Progress tracking utilities for file upload/download UIs.

Implementation

1. Add onDownloadProgress callback option
2. Wrap response.body ReadableStream to track bytes received
3. Report { loaded, total, percentage } (total from Content-Length if available)
4. Optional: upload progress via custom ReadableStream body

Files

• src/request/RequestInterceptor.ts
• tests/request/RequestInterceptor.test.ts

Source: AUDIT4 (-1 Feature-Completeness)