feat: add deterministic preview rendering

src/docforge/changesets.py

@@ -19,7 +19,7 @@ from .changeset_contract import (
     validate_id,
 )
 from .errors import DocForgeError
-from .models import Node, ProjectSnapshot, ProposalWriter
+from .models import Edge, Node, ProjectSnapshot, ProposalWriter
 from .project import Project, project_root_fingerprint
 from .proposal_projection import ProposalProjector
 
@@ -250,6 +250,21 @@ class ChangesetStore:
                 )
             return self._result(snapshot, document, valid=True, changes=changes)
 
+    def projected_snapshot(self, changeset_id: str) -> tuple[ProjectSnapshot, str]:
+        """Return a validated in-memory proposal projection for derived preview use."""
+
+        validate_id(changeset_id, "changeset_id")
+        with self._lock():
+            snapshot, document, nodes, edges = self._validate_locked(changeset_id)
+            projected = ProjectSnapshot(
+                descriptor=snapshot.descriptor,
+                nodes=tuple(sorted(nodes.values(), key=lambda node: node.node_id)),
+                edges=tuple(Edge(*edge) for edge in sorted(edges)),
+                source_hash=snapshot.source_hash,
+                revision=snapshot.revision,
+            )
+            return projected, document_hash(document)
+
     def _append(
         self,
         changeset_id: str,
@@ -409,6 +424,8 @@ class ChangesetStore:
             raise DocForgeError("path_escape", "Changeset files may not be symbolic links")
         if not path.is_file():
             raise DocForgeError("missing_changeset", "Changeset does not exist", path=path.name)
+        if path.stat().st_size > self.project.descriptor.limits.max_changeset_bytes:
+            raise DocForgeError("changeset_too_large", "Changeset exceeds configured size limit")
         raw = path.read_bytes()
         if len(raw) > self.project.descriptor.limits.max_changeset_bytes:
             raise DocForgeError("changeset_too_large", "Changeset exceeds configured size limit")