Extract generic trusted proxy handling into xff/ (stdlib only),
Echo framework adapter into xff/echo/, and slim xff/fastlyxff/
down to Fastly JSON loading.
Key changes:
- xff/ uses netip.Prefix for efficient IP matching
- Fix XFF extraction to walk right-to-left per MDN spec
- Remove echo dependency from core xff package
- fastlyxff.New() now returns *xff.TrustedProxies
