feat(audit): add FunctionRef dataclass (frozen, 4 fields)

fqname, file, line, role. Used in ProducerConsumerGraph edges
and per-aggregate producer/consumer lists. Per error_handling.md
Pattern 1 (immutability for cross-thread safety).
2 unit tests passing.

src/code_path_audit.py

@@ -9,6 +9,7 @@ postfix DSL + markdown + prefix tree text. See
 conductor/tracks/code_path_audit_20260607/spec_v2.md.
 """
 from __future__ import annotations
+from dataclasses import dataclass
 from typing import Literal
 
 AggregateKind = Literal[
@@ -51,4 +52,11 @@ RecommendedDirection = Literal[
  "unify",
  "hold",
  "insufficient_data",
-]
+]
+
+@dataclass(frozen=True)
+class FunctionRef:
+ fqname: str
+ file: str
+ line: int
+ role: str