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.md

@@ -0,0 +1,21 @@
+---
+name: fetch_http_headers
+kind: function
+lang: go
+domain: cybersecurity
+version: "1.0.0"
+purity: impure
+signature: "func FetchHTTPHeaders(url string) (map[string][]string, error)"
+description: "Realiza una solicitud HTTP HEAD a una URL y devuelve los headers de la respuesta."
+tags: [cybersecurity, io, http, headers]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports: [fmt, net/http, time]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/cybersecurity/fetch_http_headers.go"
+---