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/is_hex.md

@@ -0,0 +1,21 @@
+---
+name: is_hex
+kind: function
+lang: go
+domain: cybersecurity
+version: "1.0.0"
+purity: pure
+signature: "func IsHex(s string) bool"
+description: "Valida si un string es una cadena hexadecimal valida (longitud par, caracteres 0-9 a-f A-F)."
+tags: [cybersecurity, validation, hex, format]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: []
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/cybersecurity/is_hex.go"
+---