phase 11: GPU-resident KV cache

- GpuKVCache: pre-allocated GPU buffers, D2D copy append at offset
- Per-head strided layout [num_kv_heads, max_seq_len, head_dim]
- Fixed critical bug: seq_len must advance AFTER all layers write
  (not inside the loop per-layer)
- GpuBuffer::copy_from_device_at for offset-based D2D copy
- Tensor::from_storage constructor for wrapping raw GPU buffers
- Exported Storage and Dims from xserv-tensor

Correctness: GPU KV cache vs CPU KV cache = 50/50 bit-identical
Performance: ~neutral (KV cache was never the main bottleneck —
reshape/merge/transpose CPU round-trips dominate for Qwen3-8B)

TTFT: 122ms, TBT: 142ms, 7.0 tok/s (marginal change from 7.3)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/xserv-model/src/lib.rs

@@ -1,8 +1,10 @@
 pub mod config;
 pub mod gpt2;
+pub mod kv_cache;
 pub mod loader;
 pub mod qwen3;
 
 pub use config::ModelConfig;
 pub use gpt2::{GPT2, KVCache};
+pub use kv_cache::GpuKVCache;
 pub use qwen3::Qwen3;