Closes #16

tests/README.md

@@ -160,3 +160,10 @@ lengths 1–64. Snapshot tests mutate retained frame, signal, and description al
 after rvalue construction, and verify unchanged lookups and metadata. Failed
 constructor/builder replacement preserves the active snapshot; clear removes its
 index while saved handles remain alive.
+
+Layout validation is also compared with independent DBC bit traversal for every
+start position through one byte beyond payload sizes 0, 1, 8, 64, and 255, and
+all lengths 1–64, for both byte orders. Zero/65-bit lengths and uint32 maximum
+inputs are rejected. Builder snapshots survive input mutation and destruction;
+failed replacement with duplicate frame or signal keys preserves the existing
+frame and signal lookups.