Function fetch_recv_into 

pub fn fetch_recv_into(handle: u32, buf: &mut [u8]) -> i64

Poll the next body chunk into a caller-provided scratch buffer.

Use this form when you want to avoid per-chunk heap allocations. Prefer fetch_recv for ergonomics in higher-level code.

Returns the number of bytes written into buf (which may be smaller than the chunk the host has queued — in which case the remainder will be returned on the next call), or one of the negative sentinels documented by the host (-1 pending, -2 EOF, -3 error, -4 unknown handle).