server: persist slot checkpoints to .ckpt sidecar (LSCKPT2)

[LuminaNAO]

Problem

llama_state_seq_save_file / llama_state_seq_load_file persist the active slot KV state, but NOT the prompt-checkpoint list — which is exactly what hybrid-arch models need to avoid cold prefill on prompt switching.

On UMA systems (AMD Strix Halo, Apple Silicon, DGX Spark) where RAM = VRAM, this means checkpoint caching actively competes with model weights and KV cache for the same limited resource.

Solution

Extends both save/restore call sites with a .ckpt sidecar that serializes the std::list<common_prompt_checkpoint> alongside the main slot blob.

Sidecar format (LSCKPT2)

magic[8]    = "LSCKPT2\0"
count       : uint64                    (number of checkpoints)
for each checkpoint:
  pos_min   : int32  (llama_pos)
  pos_max   : int32
  n_tokens  : int64
  size_tgt  : uint64
  size_dft  : uint64                    (zero if no draft / no MTP)
  bytes[size_tgt]
  bytes[size_dft]

Benefits

• Hybrid/recurrent models (Qwen 3.5, etc.) avoid cold prefill on prompt switch because checkpoint lists survive slot save/restore cycles
• UMA systems eliminate the trade-off between checkpoint count and available context/layers
• Multi-user / multi-agent setups benefit from near-unlimited checkpoint persistence without consuming unified memory

Testing

Tested on AMD Strix Halo (Ryzen AI Max+ 395, 128 GB unified memory) with Qwen3.5-122B-A10B (Q5_K_XL, ~85 GB) at 220k context via Vulkan backend.

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Closes #20697

Note: This is complementary to PR #17428 (which creates checkpoints during prompt processing). This PR persists existing checkpoints to disk — the two features work together.

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