chore: declare SEVERITY_LEVELS in pz_parser.__all__

Constant was already imported by pz_classify; this just formalises
it as part of the public surface so __all__ matches actual usage.

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

tools/pz-analyzer/pz_parser.py

@@ -773,4 +773,5 @@ __all__ = [
     "MAX_STACK_FRAMES",
     "STACK_WALK_LINES",
     "MAX_CAUSE_CHAIN_LEVELS",
+    "SEVERITY_LEVELS",
 ]