feat(validators): add Check protocol and base types

Define the Check protocol for validation checks that compute a score
and return pass/fail results with diagnostics.

src/veritext/validators/base.py

@@ -0,0 +1,31 @@
+"""Base types and protocols for validation checks."""
+
+from typing import Protocol, runtime_checkable
+
+from veritext.core.types import CheckResult, ValidationContext
+
+
+@runtime_checkable
+class Check(Protocol):
+    """Protocol for validation checks.
+
+    A Check computes a score or property of text and compares it against
+    a threshold to produce a pass/fail result.
+    """
+
+    @property
+    def name(self) -> str:
+        """Return the name of this check."""
+        ...
+
+    def check(self, text: str, context: ValidationContext) -> CheckResult:
+        """Run the check and return a result.
+
+        Args:
+            text: The text to validate.
+            context: Validation context containing reference text and metadata.
+
+        Returns:
+            CheckResult with pass/fail status and diagnostics.
+        """
+        ...