fix: 12 bug fixes from comprehensive review — 51 tok/s verified on RTX 5090

P0 fixes (blocking usability):
- FIX-01: thread-local cuBLAS handle (was creating/destroying per matmul)
- FIX-16: EOS token no longer leaks into API responses
- FIX-17: max_seq_len configurable via --max-seq-len (default 2048, was hardcoded 256)
- FIX-18: max_tokens clamped to available seq space, prompt overflow returns 400

P1 fixes (bugs & performance):
- FIX-07: CachingAllocator wired into all hot paths (to_device, embedding, rope, concat)
- FIX-08: CudaDeviceProp buffer increased to 32KB for CUDA 12.9 safety
- FIX-09: tokenizer byte_fallback graceful degradation (was panic)
- FIX-19: causal mask uses -INFINITY instead of -1e9 (BF16 supports inf)
- FIX-20: LayerNorm rewritten to numerically stable two-pass algorithm
- FIX-21: min block size guard (32 threads) for LayerNorm/RMSNorm launches

P2 fixes (improvements):
- FIX-22: Option<GpuKVCache> + take() eliminates dummy KV cache allocations
- FIX-23: RoPE cache no longer artificially capped at 8192 positions

Verified on dash5 (RTX 5090): 51 tok/s batch=1, 74 tok/s 2-concurrent, 1.7-3.3x HF transformers.

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

crates/xserv-tensor/src/storage.rs

@@ -73,7 +73,7 @@ impl Storage {
         match (current, target) {
             (Device::Cpu, Device::Cuda(dev)) => {
                 let cpu_data = self.as_cpu_bytes();
-                let mut buf = GpuBuffer::alloc(cpu_data.len())?;
+                let mut buf = xserv_cuda::allocator::cached_alloc(cpu_data.len())?;
                 buf.copy_from_host(cpu_data)?;
                 Ok(Storage::cuda(buf, dev))
             }
@@ -85,7 +85,7 @@ impl Storage {
             }
             (Device::Cuda(_), Device::Cuda(dev)) => {
                 let src = self.gpu_buffer();
-                let mut dst = GpuBuffer::alloc(src.len())?;
+                let mut dst = xserv_cuda::allocator::cached_alloc(src.len())?;
                 dst.copy_from_device(src)?;
                 Ok(Storage::cuda(dst, dev))
             }
@@ -98,7 +98,7 @@ impl Storage {
         match self.0.as_ref() {
             StorageInner::Cpu { data } => Ok(Storage::cpu(data.clone())),
             StorageInner::Cuda { buffer, device } => {
-                let mut dst = GpuBuffer::alloc(buffer.len())?;
+                let mut dst = xserv_cuda::allocator::cached_alloc(buffer.len())?;
                 dst.copy_from_device(buffer)?;
                 Ok(Storage::cuda(dst, *device))
             }