Candle bridge for KAIO — CustomOp bindings for 12 GPU ops (matmul_tc, matmul_tc_bf16, matmul_tc_async, matmul_tc_bf16_async, matmul_int4, matmul_int8, attention_tc, attention_tc_causal, attention_flash, attention_flash_causal, qkv_project_int8, qkv_project_int4). All four matmul TC variants (f16+bf16, sync+async) support backward (autograd); FlashAttention (plain+causal) supports backward via dedicated PTX kernels. Build with `cargo build --features cuda`.