server: on-demand VRAM sharing to time-share one GPU between models

Add release/restore of a model's GPU weight buffers (keeping a host shadow
and the KV cache) so several always-loaded llama-server processes can
time-share a single GPU without reloading or losing the prompt cache.

- llama-model: release_device_weights()/restore_device_weights() capture a
  compact host shadow (stable iteration order, view-skipping) and free then
  realloc the device weight buffers; weights_resident() query.
- llama-context: release_device()/restore_device() wrappers; decode() auto-
  restores; public C API llama_context_release_device/restore_device.
- server: LLAMA_SLEEP_VRAM_ONLY makes idle-sleep release only the VRAM weights
  (not a full unload/reload). A cross-process flock token in LLAMA_VRAM_ARENA
  enforces "resident iff holds token"; an inotify doorbell forces the holder
  to release on contention. The warden thread only touches the task queue, so
  releases run on the loop thread and never race a decode.

Validated on RX 580 (Vulkan): two models share 8GB, never both resident,
correct output under contention, KV cache preserved (no re-prefill).

Assisted-by: Claude

tools/server/server-queue.cpp

@@ -116,6 +116,12 @@ void server_queue::wait_until_no_sleep() {
     }
 }
 
+void server_queue::request_yield() {
+    std::unique_lock<std::mutex> lock(mutex_tasks);
+    yield_requested = true;
+    condition_tasks.notify_all();
+}
+
 void server_queue::terminate() {
     std::unique_lock<std::mutex> lock(mutex_tasks);
     running = false;
@@ -129,6 +135,9 @@ void server_queue::start_loop(int64_t idle_sleep_ms) {
     constexpr auto max_wait_time = std::chrono::seconds(1);
     auto should_sleep = [&]() -> bool {
         // caller must hold mutex_tasks
+        if (yield_requested) {
+            return true; // another process rang the VRAM doorbell - release now
+        }
         if (idle_sleep_ms < 0) {
             return false;
         }
@@ -178,6 +187,7 @@ void server_queue::start_loop(int64_t idle_sleep_ms) {
             if (should_sleep()) {
                 QUE_INF("%s", "entering sleeping state\n");
                 sleeping = true;
+                yield_requested = false; // consumed
                 callback_sleeping_state(true);
                 req_stop_sleeping = false;
                 // wait until we are requested to exit sleeping state
@@ -195,13 +205,17 @@ void server_queue::start_loop(int64_t idle_sleep_ms) {
                 condition_tasks.notify_all(); // notify wait_until_no_sleep()
                 break; // process new tasks
             } else {
-                // wait for new tasks or timeout for checking sleeping condition
+                // wait for new tasks, a VRAM yield request, or timeout for checking sleeping condition
                 bool res = condition_tasks.wait_for(lock, max_wait_time, [&]{
-                    return (!queue_tasks.empty() || !running);
+                    return (!queue_tasks.empty() || !running || yield_requested);
                 });
-                if (res) {
+                if (res && !queue_tasks.empty()) {
                     break; // new task arrived or terminate
                 }
+                if (!running) {
+                    break;
+                }
+                // otherwise (timeout or yield request), loop again to re-check should_sleep
                 // otherwise, loop again to check sleeping condition
             }
         }