Add Caesar cipher project with Doxygen docs

src/caesar_cipher/cipher.py

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+## @file cipher.py
+#  @brief Caesar cipher encryption.
+
+from caesar_cipher.constants import ALPHABET
+
+
+def encrypt(text: str, shift: int) -> str:
+    ## @brief Encrypt a text with the Caesar cipher.
+    #  @param text  Original text.
+    #  @param shift Positions to move each letter (may be negative or > 26).
+    #  @return The encrypted text; non-letters are kept, case is preserved.
+    result = []
+    for char in text:
+        lower = char.lower()
+        if lower in ALPHABET:
+            new_position = (ALPHABET.index(lower) + shift) % len(ALPHABET)
+            new_char = ALPHABET[new_position]
+            result.append(new_char.upper() if char.isupper() else new_char)
+        else:
+            result.append(char)
+    return "".join(result)