server: optionally evict the KV cache too (Phase 2 of VRAM sharing)

Extend on-demand device residency to the KV cache so that when a model's KV
plus another model would not fit in VRAM, the KV can also be evicted to a host
shadow (D2H on release, H2D on restore) instead of only the weights.

- llama_memory_i: add release_device_buffers()/restore_device_buffers()
  (default no-op). Implemented in llama_kv_cache (D2H shadow of the live
  ctxs_bufs, freed and reallocated like the weights); llama_memory_hybrid and
  llama_kv_cache_iswa delegate to their child caches.
- llama_context::release_device(evict_kv): also evict the memory's device
  buffers when requested; restore_device() rebuilds them. Public API
  llama_context_release_device gains an evict_kv flag.
- server: LLAMA_SLEEP_EVICT_KV=1 enables it. Off by default (weights-only),
  since the KV shadow adds a D2H/H2D copy of the live cache each cycle.

Validated on RX 580 (Vulkan), 4B @ 32k ctx: weights-only cold VRAM 1750 MB
(KV stays); weights+KV cold VRAM 726 MB (KV freed, ~1 GB reclaimed). KV
survives the round-trip: prompt cache reused after the cycle (prompt_n 4 vs
42), correct output.

Assisted-by: Claude

src/llama-context.h

@@ -59,8 +59,10 @@ struct llama_context {
 
     // on-demand device (VRAM) residency: free / rebuild the model's GPU weight buffers so that
     // VRAM can be time-shared with another model. release_device() synchronizes first; decode()
-    // auto-restores when it finds the weights have been released.
-    void release_device();
+    // auto-restores when it finds the weights have been released. If evict_kv is set, the KV cache
+    // device buffers are also evicted to a host shadow (for when the KV would not fit alongside the
+    // other model) - this adds a D2H/H2D copy of the live KV on each cycle.
+    void release_device(bool evict_kv = false);
     void restore_device();
 
     const llama_model   & get_model()   const;
@@ -351,6 +353,9 @@ private:
 
     bool sched_need_reserve = true;
 
+    // on-demand device residency: true while the KV cache device buffers have been evicted to host
+    bool kv_device_evicted = false;
+
     ggml_backend_t backend_cpu = nullptr;
     std::vector<ggml_backend_ptr> backends;