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

tools/server/server-context.cpp

@@ -914,6 +914,7 @@ private:
     // weights) right before it decodes, and goes cold (releases weights + unlocks) when it idles,
     // so only one model holds VRAM at a time and the KV cache is never evicted (no re-prefill).
     bool vram_only    = false;   // release-mode sleep active (LLAMA_SLEEP_VRAM_ONLY)
+    bool vram_evict_kv = false;  // also evict the KV cache to host on release (LLAMA_SLEEP_EVICT_KV)
     bool vram_flock   = false;   // cross-process flock coordination available
     std::atomic<bool> vram_cold{true}; // weights currently released (read by warden thread)
     int  vram_lock_fd = -1;      // fd for <arena>/token.lock
@@ -931,6 +932,7 @@ private:
             return;
         }
         vram_only = true;
+        vram_evict_kv = getenv("LLAMA_SLEEP_EVICT_KV") != nullptr;
         vram_cold = false; // weights are resident right after load
 #if !defined(_WIN32)
         const char * arena_env = getenv("LLAMA_VRAM_ARENA");
@@ -1040,9 +1042,9 @@ private:
         if (!vram_only || vram_cold) {
             return;
         }
-        llama_context_release_device(ctx_tgt);
+        llama_context_release_device(ctx_tgt, vram_evict_kv);
         if (ctx_dft != nullptr) {
-            llama_context_release_device(ctx_dft);
+            llama_context_release_device(ctx_dft, vram_evict_kv);
         }
 #if !defined(_WIN32)
         if (vram_flock) {