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

src/llama-context.h

@@ -57,6 +57,12 @@ struct llama_context {
 
     void synchronize();
 
+    // 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();
+    void restore_device();
+
     const llama_model   & get_model()   const;
     const llama_cparams & get_cparams() const;