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-memory-hybrid.h

@@ -76,6 +76,9 @@ public:
     void state_write(llama_io_write_i & io, llama_seq_id seq_id = -1, llama_state_seq_flags flags = 0) const override;
     void state_read (llama_io_read_i  & io, llama_seq_id seq_id = -1, llama_state_seq_flags flags = 0)       override;
 
+    void release_device_buffers() override;
+    bool restore_device_buffers() override;
+
     //
     // llama_memory_hybrid specific API
     //