CAD recipe v2: legible version-mismatch errors, refuse old files cleanly

Port of snaporca 04c3d579a7. Bump SNAPORCA_CAD_RECIPE_VERSION 1 -> 2;
deserialize_recipe sets a user-facing error distinguishing too-new / too-old
/ corrupt instead of a silent bare false. No per-version migration by design.

Golden fixture regenerated at v2 (v1 retired), field-value reorder tripwire
unchanged. Fork adjustment: Catch2 v3 string matcher ContainsSubstring
(not v2's Contains) in the two new version-mismatch tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/libslic3r/CadDocument.cpp

@@ -1775,18 +1775,35 @@ std::string CadDocument::serialize_recipe() const
 
 bool CadDocument::deserialize_recipe(const std::string& blob)
 {
+    error.clear();
     try {
         std::istringstream iss(blob);
         cereal::BinaryInputArchive ar(iss);
         uint32_t v;
         ar(v);
-        if (v > SNAPORCA_CAD_RECIPE_VERSION)
+        if (v > SNAPORCA_CAD_RECIPE_VERSION) {
+            error = "saved with a newer version of SnapOrca CAD (format v"
+                  + std::to_string(v) + ", this build reads up to v"
+                  + std::to_string(SNAPORCA_CAD_RECIPE_VERSION) + ")";
             return false;
+        }
+        if (v < SNAPORCA_CAD_RECIPE_VERSION) {
+            error = "saved with an older version of SnapOrca CAD (format v"
+                  + std::to_string(v) + "); this project cannot be opened by this build";
+            return false;
+        }
         ar(features);
         return recompute();
-    } catch (const Standard_Failure&) {
+    } catch (const Standard_Failure& e) {
+        const char* what = e.GetMessageString();
+        error = std::string("CAD data could not be read")
+              + (what && *what ? ": " + std::string(what) : "");
+        return false;
+    } catch (const std::exception& e) {
+        error = std::string("CAD data could not be read: ") + e.what();
         return false;
     } catch (...) {
+        error = "CAD data could not be read";
         return false;
     }
 }

src/libslic3r/CadDocument.hpp

@@ -356,7 +356,11 @@ public:
     void clear();
     bool recompute();   // replay features -> body + display_mesh; false on error
 
-    static constexpr uint32_t SNAPORCA_CAD_RECIPE_VERSION = 1;
+    // CadRecipe serialization contract:
+    // - bump this whenever CadFeature::save/load gains or loses a field
+    // - v1 blobs are deliberately not loadable; there is no migration path by design
+    // - append fields ONLY at the end of save/load, never reorder (golden fixture enforces this)
+    static constexpr uint32_t SNAPORCA_CAD_RECIPE_VERSION = 2;
     std::string serialize_recipe() const;
     bool deserialize_recipe(const std::string& blob);