feat(core): add tokenisation with unicode support

Implement Tokeniser protocol and WordTokeniser class with NFC Unicode
normalisation, optional lowercasing, and punctuation removal.

src/veritext/core/tokenisation.py

@@ -0,0 +1,81 @@
+"""Tokenisation utilities for text processing."""
+
+import re
+import unicodedata
+from typing import Literal, Protocol
+
+NormalisationForm = Literal["NFC", "NFD", "NFKC", "NFKD"]
+
+
+class Tokeniser(Protocol):
+    """Protocol for text tokenisers."""
+
+    def tokenise(self, text: str) -> list[str]:
+        """
+        Tokenise text into a list of tokens.
+
+        Args:
+            text: The text to tokenise.
+
+        Returns:
+            List of tokens.
+        """
+        ...
+
+
+class WordTokeniser:
+    """
+    Word-level tokeniser with Unicode normalisation.
+
+    Splits text into words, with options for lowercasing and punctuation removal.
+    """
+
+    def __init__(
+        self,
+        lowercase: bool = True,
+        remove_punctuation: bool = True,
+        normalisation_form: NormalisationForm = "NFC",
+    ) -> None:
+        """
+        Initialise the tokeniser.
+
+        Args:
+            lowercase: Whether to convert tokens to lowercase.
+            remove_punctuation: Whether to remove punctuation from tokens.
+            normalisation_form: Unicode normalisation form (NFC, NFD, NFKC, NFKD).
+        """
+        self.lowercase = lowercase
+        self.remove_punctuation = remove_punctuation
+        self.normalisation_form: NormalisationForm = normalisation_form
+
+        # Pattern for punctuation removal (keeps alphanumeric and Unicode letters)
+        self._punctuation_pattern = re.compile(r"[^\w\s]", re.UNICODE)
+
+    def tokenise(self, text: str) -> list[str]:
+        """
+        Tokenise text into words.
+
+        Args:
+            text: The text to tokenise.
+
+        Returns:
+            List of word tokens. Empty list if text is empty or whitespace-only.
+        """
+        if not text or not text.strip():
+            return []
+
+        # Unicode normalisation
+        normalised = unicodedata.normalize(self.normalisation_form, text)
+
+        # Lowercase if requested
+        if self.lowercase:
+            normalised = normalised.lower()
+
+        # Remove punctuation if requested
+        if self.remove_punctuation:
+            normalised = self._punctuation_pattern.sub(" ", normalised)
+
+        # Split on whitespace and filter empty strings
+        tokens = normalised.split()
+
+        return tokens