[RT-E5] A byte-level SSE reader, with no UTF-16 round trip #484

Part of #350, the quality-refactoring roadmap (road to 1.0, step 3).

Goal: A byte-level SSE reader, with no UTF-16 round trip (about 3× less allocation).

Lane: Runtime. Status: todo.

Every PR of this epic starts its title with [RT-E5]. The finding issues it closes are linked as sub-issues; this issue closes when the epic's last PR merges and master CI is green.