Per-tier packet framing state machine.

Bounded buffers

Every Accumulator implementation should support a :max_size option limiting how much data or how many whole packets may be held before signalling overflow. Because push/3 returns only acc(), implementations typically record overflow during push/3 (for example by dropping the oldest entry and setting a flag) and surface it once from the next pop/1 as {:error, :buffer_overflow, acc}. The engine logs :frame_error telemetry and continues draining after overflow.