mtmd: release/restore the encoder weights from VRAM on demand

Add clip_release_device/clip_restore_device (and mtmd_release_device/
mtmd_restore_device wrappers over the vision + audio contexts) that free
the multimodal encoder's device weight buffer to a read-only host shadow
and rebuild it on demand, using the same shadow/free/reallocate pattern
as llama_model weights. No-op for a CPU-backed encoder. This lets the
server drop the ~hundreds-of-MiB vision encoder from VRAM when it is not
encoding an image.

Assisted-by: Claude

tools/mtmd/clip.h

@@ -67,6 +67,12 @@ struct clip_init_result clip_init(const char * fname, struct clip_context_params
 
 void clip_free(struct clip_ctx * ctx);
 
+// on-demand device (VRAM) residency: free/rebuild the encoder weight buffer to shrink an idle
+// (cold) multimodal model's VRAM footprint. No-op for a CPU-backed encoder. See clip.cpp.
+void clip_release_device(struct clip_ctx * ctx);
+bool clip_restore_device(struct clip_ctx * ctx);
+bool clip_weights_resident(const struct clip_ctx * ctx);
+
 // TODO: should be enum, not string
 const char * clip_patch_merge_type(const struct clip_ctx * ctx);