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_test.go

@@ -0,0 +1,31 @@
+package xffecho
+
+import (
+	"testing"
+
+	"go.ntppool.org/common/xff"
+)
+
+func TestTrustOptions(t *testing.T) {
+	tp, err := xff.NewFromCIDRs([]string{
+		"192.0.2.0/24",
+		"203.0.113.0/24",
+		"2001:db8::/32",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	opts := TrustOptions(tp)
+	if len(opts) != 3 {
+		t.Errorf("expected 3 trust options, got %d", len(opts))
+	}
+}
+
+func TestTrustOptionsEmpty(t *testing.T) {
+	tp := xff.New()
+	opts := TrustOptions(tp)
+	if len(opts) != 0 {
+		t.Errorf("expected 0 trust options, got %d", len(opts))
+	}
+}