Preserve semantic reader after failed generation rebuild

docs/SEMANTIC.md

@@ -118,7 +118,10 @@ dispatches supported CPU SIMD kernels with portable fallback. The Cargo config
 contains the Windows NK_DYNAMIC override required for static NumKong linkage.
 
 Staging vectors and a real HNSW graph are persisted with an ANN blob SHA256.
-Activation is atomic. Failed rebuilds preserve the active generation; the next
+Activation is atomic. Failed rebuilds preserve the active generation and the
+same search session's reader policy, including failures acquiring the build
+lock, starting a generation, publishing it or discarding failed staging. The
+original failure remains visible if cleanup also fails. The next
 rebuild reclaims abandoned staging under a cross-process lock. Search uses a
 consistent read snapshot.