Design a PacketAssembler API for Streaming Fragments

Context and Assumptions

• You are building a small component that assembles packets from a byte stream.
• Each packet begins with a 1-byte header that encodes the payload length N (0–255). The header byte is not part of the payload.
• The stream may arrive fragmented arbitrarily; a single call may contain parts of a packet or multiple full packets back-to-back.

Example interaction:

• packetit(0x04, 'a', 'b', 'c') -> no output (header 4 indicates a 4-byte payload; only 3 bytes arrived)
• packetit('d') -> 'abcd' (completes the prior packet)

Requirements

1. Specify the class and method signatures of a PacketAssembler API.
2. Describe state management: buffering, expected size, and how fragments are handled.
3. Behavior when receiving multiple consecutive packets within one or many calls.
4. Handling of invalid headers and oversized input (e.g., header value exceeds a configured maximum).
5. Define when/how state resets after emission and on errors.
6. Thread-safety considerations and options.
7. Provide tests (unit-style examples) that exercise common and edge cases.
8. Analyze time and space complexity.