integration testing (10, 100, 600 bytes) (infra)

[Noah4649]

Test the full pipeline with a 10-byte message (matches spec output exactly), a 100-byte message, and a 600-byte message (triggers segmentation). Verify correctness at every layer.

[DigBatt]

Think I've done it. Seems to look ok. For instance 600 byte is broken up into 500 and 100 byte segments