Bound X-Forwarded-For scanning allocation to the hop cap (closes #133)

forwardedClientAddr now walks the header values in reverse with strings.LastIndexByte instead of joining and splitting, so allocation is bounded by the 64-hop cap rather than by header length: 1.6 MB per call becomes 16 bytes for a 1 MB chain. Semantics are unchanged, verified by differential testing against the previous implementation.

internal/middleware/export_test.go

@@ -25,6 +25,11 @@ func IPFromHostPort(hp string) string {
 	return ipFromHostPort(hp)
 }
 
+// ClientKeyForTest exposes clientKey for testing.
+func ClientKeyForTest(m *Middleware, r *http.Request) string {
+	return m.clientKey(r)
+}
+
 // IsClientTLS exposes isClientTLS for testing.
 func IsClientTLS(r *http.Request) bool {
 	return isClientTLS(r)