b4db953 committed before a green build: the existing
launch_decode_attention_bf16 call wasn't updated for the new kernel
signature (too-few-args at flash_attention.cu:433) and the Rust FFI
binding + decode_attention_sink wrapper never landed (failed string
matches). This adds the missing pieces; xserv now builds green on dash5
(release, 17s). qwen3 decode path unchanged (passes nullptr/0).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>