Complete DFG-8 adapter proposal support

src/docforge/models.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Mapping
 from dataclasses import asdict, dataclass
 from pathlib import Path
 from typing import Protocol
@@ -128,6 +129,13 @@ class ProjectService(Protocol):
 
     def canonical_source_paths(self) -> tuple[Path, ...]: ...
 
+    def validate_proposal(
+        self,
+        base: ProjectSnapshot,
+        projected: ProjectSnapshot,
+        operations: tuple[Mapping[str, object], ...],
+    ) -> None: ...
+
 
 @dataclass(frozen=True)
 class ContextEntry: