feat: add shadow traffic diff mode

[onoja123]

Summary

This PR introduces Shadow Traffic Diff mode for HTTP tunnels, allowing requests to be mirrored to a shadow target and automatically compared against the primary response.

Key highlights:

• Adds shadow traffic + diffing support (mirror requests to a local/remote target and log divergences).
• Introduces CLI flags and TOML configuration for shadow settings.
• Exposes shadow options in Vite and Next.js plugins.
• Documents usage across CLI, observability, and plugin guides.

Details

• Shadow requests run fully in parallel and never affect the primary response.
• Diffs are emitted only when status, body, or selected headers diverge.
• Includes sampling, timeout handling, and body size caps for safety and performance.
• Designed to safely validate new services or changes in production without impacting live traffic.

Testing

Local smoke test:

• Start primary server on port 3000
• Start shadow server on port 4000

Example:

index.js http 3000 --shadow-port 4000

Verified mirrored requests and confirmed diffs are logged when responses differ.

[vercel]

@onoja123 is attempting to deploy a commit to the Akinkunmi Team on Vercel.

A member of the Team first needs to authorize it.