feat: 16 funciones cybersecurity — análisis, crypto e IO de seguridad

12 funciones puras con implementación real:
HashSHA256, HashMD5, EntropyShannon, IsBase64, IsHex, ExtractURLs,
ParseIPCIDR, IPInRange, NormalizeURL, DetectSQLInjection,
LevenshteinDistance, JaccardSimilarity

4 funciones impuras con implementación real (stdlib):
LookupWhois, ResolveDNS, FetchHTTPHeaders, ScanPortTCP

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/cybersecurity/fetch_http_headers.go

@@ -0,0 +1,27 @@
+package cybersecurity
+
+import (
+	"fmt"
+	"net/http"
+	"time"
+)
+
+// FetchHTTPHeaders realiza una solicitud HTTP HEAD a la URL y devuelve los headers de respuesta.
+func FetchHTTPHeaders(url string) (map[string][]string, error) {
+	client := &http.Client{
+		Timeout: 10 * time.Second,
+	}
+
+	resp, err := client.Head(url)
+	if err != nil {
+		return nil, fmt.Errorf("error realizando solicitud HEAD a %s: %w", url, err)
+	}
+	defer resp.Body.Close()
+
+	headers := make(map[string][]string, len(resp.Header))
+	for k, v := range resp.Header {
+		headers[k] = v
+	}
+
+	return headers, nil
+}