Add zero-alloc wire-format peek helpers

[cvik]

Summary

• Add Packet.peekKind, peekMsgId, peekOption for extracting header fields and scanning options from raw wire bytes without allocating
• Add Packet.emptyAck, emptyRst for constructing minimal 4-byte response packets
• Enables higher-level libraries (coap server/client) to avoid inline bit manipulation for hot-path wire-format access

Test plan

• All existing tests pass
• New tests for each helper: peekKind, peekMsgId, peekOption (hit, miss, extended delta, truncated data), emptyAck/emptyRst round-trip through Packet.read