Add Caesar cipher project with Doxygen docs

src/caesar_cipher/__init__.py

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+## @file __init__.py
+#  @brief Public API of the caesar_cipher package.
+
+from caesar_cipher.cipher import encrypt
+
+__all__ = ["encrypt"]

src/caesar_cipher/__main__.py

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+## @file __main__.py
+#  @brief Enables `python -m caesar_cipher`.
+
+from caesar_cipher.main import main
+
+main()

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)

src/caesar_cipher/constants.py

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+## @file constants.py
+#  @brief Constants used by the Caesar cipher.
+
+## Lowercase alphabet used to look up letter positions.
+ALPHABET = list("abcdefghijklmnopqrstuvwxyz")
+
+## Prompt for the message.
+PROMPT_TEXT = "Type your message:\n"
+## Prompt for the shift number.
+PROMPT_SHIFT = "Type the shift number:\n"
+
+## Name of the environment file holding tokens.
+ENV_FILE = ".env"

src/caesar_cipher/env.py

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+## @file env.py
+#  @brief Minimal stdlib `.env` reader (no third-party dependency).
+
+import os
+from pathlib import Path
+
+from caesar_cipher.constants import ENV_FILE
+
+
+def load_env(path: str | Path = ENV_FILE) -> dict[str, str]:
+    ## @brief Read KEY=VALUE pairs from an env file.
+    #  @param path Path of the env file.
+    #  @return Mapping of variables; empty if the file does not exist.
+    values: dict[str, str] = {}
+    env_path = Path(path)
+    if not env_path.is_file():
+        return values
+    for line in env_path.read_text(encoding="utf-8").splitlines():
+        line = line.strip()
+        if not line or line.startswith("#") or "=" not in line:
+            continue
+        key, _, value = line.partition("=")
+        values[key.strip()] = value.strip().strip("\"'")
+    return values
+
+
+def get_token(name: str, path: str | Path = ENV_FILE) -> str | None:
+    ## @brief Get a token from the process environment or the env file.
+    #  @param name Variable name, e.g. GITEA_TOKEN.
+    #  @return The token or None when not set.
+    return os.environ.get(name) or load_env(path).get(name)

src/caesar_cipher/main.py

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+## @file main.py
+#  @brief Interactive command line interface.
+
+from caesar_cipher import constants as c
+from caesar_cipher.cipher import encrypt
+
+
+def main() -> None:
+    ## @brief Ask for a message and a shift, then print the encrypted text.
+    text = input(c.PROMPT_TEXT)
+    try:
+        shift = int(input(c.PROMPT_SHIFT))
+    except ValueError:
+        print("The shift must be a whole number.")
+        return
+    print(f"Here's the encoded result: {encrypt(text, shift)}")
+
+
+if __name__ == "__main__":
+    main()