Function at_serialized_capacity 

pub(crate) fn at_serialized_capacity<'a, A>(borrow: &A) -> bool
where
    A: AsBytes<'a>,

Returns true once the serialized size of borrow reaches 10% under SHIP_WORDS.

Monotone in size, deliberately not a window below the boundary. A single record wider than a window steps clear over it, and a ship signal that un-fires past the boundary lets a chunk grow until it exceeds the buffer pool’s largest size class, past which a spilled body degrades to permanently resident. The same heuristic as builder::ColumnBuilder’s ship point, lifted out so the builder, the merger, and the SizableContainer impl agree on the signal.