fix: address code review findings on pz_parser

- Strip body-prefix severity in normalize_first_line so pattern_id
  is stable across body-prefix vs bracketed-only variants.
- Lookback for inferred attribution now counts raw file lines
  (per spec literal), not body-line budget across entries.
- Document hash truncation (64-bit) and direct-attribution priority.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/pz-analyzer/tests/test_signatures.py

@@ -55,5 +55,37 @@ class SignatureUniquenessTests(unittest.TestCase):
         self.assertEqual(pat[:7], "sha256:")
 
 
+class SeverityPrefixStripTests(unittest.TestCase):
+    """A body line that begins with a literal severity word (``SEVERE:``,
+    ``ERROR:``, ``WARN:``, ``FATAL:``) should not fragment pattern_id away
+    from the otherwise-identical body that lacks the prefix. The bracketed
+    level already feeds pattern_id; the prefix is redundant and varies in
+    practice."""
+
+    def test_pattern_id_invariant_under_body_prefix_severe(self) -> None:
+        # Same logical error: one line carries ``SEVERE: `` body prefix, the
+        # other doesn't. Both classified as SEVERE by their bracketed level.
+        with_prefix = pz_parser.compute_pattern_id(
+            "SEVERE",
+            "SEVERE: foo at zombie.X(File.java:42)",
+        )
+        without_prefix = pz_parser.compute_pattern_id(
+            "SEVERE",
+            "foo at zombie.X(File.java:42)",
+        )
+        self.assertEqual(with_prefix, without_prefix)
+
+    def test_pattern_id_invariant_under_body_prefix_error(self) -> None:
+        with_prefix = pz_parser.compute_pattern_id(
+            "ERROR",
+            "ERROR: doStuff failed in module",
+        )
+        without_prefix = pz_parser.compute_pattern_id(
+            "ERROR",
+            "doStuff failed in module",
+        )
+        self.assertEqual(with_prefix, without_prefix)
+
+
 if __name__ == "__main__":
     unittest.main()