llama: evict recurrent/SSM state on device release

The recurrent (SSM/conv) state of hybrid models (e.g. Qwen3.5) was left
resident when a model's device buffers were released for on-demand VRAM
sharing - llama_memory_recurrent::release_device_buffers() was a no-op
default. Implement it (and restore_device_buffers) with the same
capture-host-shadow / free / reallocate pattern as llama_kv_cache, so
llama_memory_hybrid now evicts both its attention KV and its recurrent
state. The state is read-write, so its shadow is recaptured on every
release.

Assisted-by: Claude

src/llama-memory-hybrid.cpp

@@ -202,7 +202,7 @@ void llama_memory_hybrid::state_read(llama_io_read_i & io, llama_seq_id seq_id,
 }
 
 void llama_memory_hybrid::release_device_buffers() {
-    // evict the attention KV (grows with context); the recurrent state uses the no-op default
+    // evict both the attention KV (grows with context) and the recurrent/SSM state
     mem_attn->release_device_buffers();
     mem_recr->release_device_buffers();
 }