kv-cache: check the mirrored slot layout on a whole-context restore too

state_read_meta only looked at the layout it was given on the single-sequence path.
A whole-context restore lays the cells out from 0 in both caches, so they agree as
long as they restore the same number of cells, but nothing checked that they did: an
indexer section belonging to some other context was read over cells the attention
cache had filled from a different one, which is the state the indexer must never be
left in.

src/llama-kv-cache.cpp

@@ -2352,6 +2352,14 @@ bool llama_kv_cache::state_read_meta(llama_io_read_i & io, uint32_t strm, uint32
             return false;
         }
 
+        // the cells go in from 0, so a mirrored cache lands on the same ones as long as it
+        // restores the same count. the layout itself carries no more information here
+        if (sinfo_in && (sinfo_in->empty() || sinfo_in->n_stream() != 1 || sinfo_in->idxs[0].size() != cell_count)) {
+            LLAMA_LOG_ERROR("%s: mirrored slot layout holds %d cells, this cache restores %d\n", __func__,
+                    sinfo_in->empty() ? 0 : (int) sinfo_in->idxs[0].size(), cell_count);
+            return false;
+        }
+
         for (uint32_t i = 0; i < cell_count; ++i) {
             llama_pos pos;
             uint32_t  n_seq_id;