Constant DEFAULT_FETCH_MEMORY_BUDGET_BYTES 

pub const DEFAULT_FETCH_MEMORY_BUDGET_BYTES: u64 = _; // 268_435_456u64

Default fetch-budget cap on transient origin-GET buffer memory (256 MiB): the smallest budget at which DEFAULT_TOTAL_FETCH_PERMITS runs of DEFAULT_MAX_COALESCED_GET_BYTES each can all be in flight, so at the defaults a cold contiguous scan is throttled by parallelism, not memory.