refactor(xff): split into generic, echo, and fastly packages

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

xff/echo/echo.go

@@ -0,0 +1,48 @@
+// Package xffecho adapts [xff.TrustedProxies] for use with the Echo web
+// framework's X-Forwarded-For IP extraction.
+//
+// # Usage
+//
+//	tp, err := fastlyxff.New("fastly.json")
+//	if err != nil {
+//		return err
+//	}
+//	trustOpts := xffecho.TrustOptions(tp)
+//	e.IPExtractor = echo.ExtractIPFromXFFHeader(trustOpts...)
+package xffecho
+
+import (
+	"net"
+	"net/netip"
+
+	"github.com/labstack/echo/v4"
+
+	"go.ntppool.org/common/xff"
+)
+
+// TrustOptions converts a [xff.TrustedProxies] into Echo trust options
+// for use with [echo.ExtractIPFromXFFHeader].
+func TrustOptions(tp *xff.TrustedProxies) []echo.TrustOption {
+	prefixes := tp.Prefixes()
+	opts := make([]echo.TrustOption, 0, len(prefixes))
+	for _, p := range prefixes {
+		opts = append(opts, echo.TrustIPRange(prefixToIPNet(p)))
+	}
+	return opts
+}
+
+// prefixToIPNet bridges netip.Prefix (used by xff) to net.IPNet (used by Echo).
+func prefixToIPNet(p netip.Prefix) *net.IPNet {
+	addr := p.Masked().Addr()
+	bits := p.Bits()
+
+	ipLen := 128
+	if addr.Is4() {
+		ipLen = 32
+	}
+
+	return &net.IPNet{
+		IP:   net.IP(addr.AsSlice()),
+		Mask: net.CIDRMask(bits, ipLen),
+	}
+}