Add Polybius square codec

Co-authored-by: dhondta <9108102+dhondta@users.noreply.github.com>
Agent-Logs-Url: https://github.com/dhondta/python-codext/sessions/d62d52ab-5a81-4502-a03c-e834bf9c65ff

Author: Copilot

src/codext/crypto/__init__.py

@@ -4,6 +4,7 @@
 from .bacon import *
 from .barbie import *
 from .citrix import *
+from .polybius import *
 from .railfence import *
 from .rot import *
 from .scytale import *

src/codext/crypto/polybius.py

@@ -0,0 +1,46 @@
+# -*- coding: UTF-8 -*-
+"""Polybius Square Codec - polybius-square content encoding.
+
+The Polybius square is a method for fractionating plaintext characters so that
+they can be represented by a smaller set of symbols. Each letter is represented
+by its coordinates (row, column) in the 5×5 grid.
+
+This codec:
+- en/decodes strings from str to str
+- en/decodes strings from bytes to bytes
+- decodes file content to str (read)
+- encodes file content from str to bytes (write)
+
+Reference: https://en.wikipedia.org/wiki/Polybius_square
+"""
+from ..__common__ import *
+
+
+__examples__ = {
+    'enc(polybius|polybius-square|polybius_square)': {
+        'this is a test': "44232443 2443 11 44154344",
+        'jack':           "24111325",
+    },
+    'dec(polybius)': {
+        '44232443 2443 11 44154344': "THIS IS A TEST",
+    },
+}
+__guess__ = ["polybius-square"]
+
+
+# Standard 5×5 Polybius square (I and J share the same cell):
+#      1  2  3  4  5
+# 1    A  B  C  D  E
+# 2    F  G  H  I  K
+# 3    L  M  N  O  P
+# 4    Q  R  S  T  U
+# 5    V  W  X  Y  Z
+_ALPHABET = "ABCDEFGHIKLMNOPQRSTUVWXYZ"
+ENCMAP = {_ALPHABET[i]: str(i // 5 + 1) + str(i % 5 + 1) for i in range(25)}
+ENCMAP['J'] = ENCMAP['I']
+ENCMAP[' '] = ' '
+
+
+add_map("polybius-square", ENCMAP, ignore_case="both",
+        pattern=r"^(?:polybius(?:[-_]square)?)$", printables_rate=1.,
+        expansion_factor=(1.85, .15))