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>

csrc/normalization/rmsnorm.cu

@@ -108,6 +108,7 @@ extern "C" {
 void launch_rmsnorm_f32(const void* x, const void* gamma, void* out,
                         int rows, int hidden_size, float eps, void* stream) {
     int block = (hidden_size < 1024) ? hidden_size : 1024;
+    if (block < 32) block = 32;
     rmsnorm_f32<<<rows, block, 0, (cudaStream_t)stream>>>(
         (const float*)x, (const float*)gamma, (float*)out, hidden_size, eps);
 }
@@ -115,6 +116,7 @@ void launch_rmsnorm_f32(const void* x, const void* gamma, void* out,
 void launch_rmsnorm_bf16(const void* x, const void* gamma, void* out,
                          int rows, int hidden_size, float eps, void* stream) {
     int block = (hidden_size < 1024) ? hidden_size : 1024;
+    if (block < 32) block = 32;
     rmsnorm_bf16<<<rows, block, 0, (cudaStream_t)stream>>>(
         (const __nv_bfloat16*)x, (const __nv_bfloat16*)gamma,
         (__nv_bfloat16*)out, hidden_size, eps);
@@ -124,6 +126,7 @@ void launch_add_rmsnorm_bf16(const void* x, const void* residual, const void* ga
                              void* normed_out, void* sum_out,
                              int rows, int hidden_size, float eps, void* stream) {
     int block = (hidden_size < 1024) ? hidden_size : 1024;
+    if (block < 32) block = 32;
     add_rmsnorm_bf16<<<rows, block, 0, (cudaStream_t)stream>>>(
         (const __nv_bfloat16*)x, (const __nv_bfloat16*)residual,
         (const __nv_bfloat16*)gamma,