KNX primitives for the public KNXyz Rust crates.

This crate currently exposes address types, shared error types, selected KNXnet/IP header primitives, and std-gated protocol helper surfaces. It is not a commissioning tool and does not perform device programming, application download, Secure commissioning, or project-file write-back.

Feature boundaries

• Default features enable std.
• --no-default-features keeps a limited no_std surface for addresses, errors, and selected KNXnet/IP decode primitives.
• serde adds optional derives for exported types without enabling std.
• std currently enables allocation-backed convenience APIs, cEMI/APCI helpers, encode helpers, and std::error::Error for KnxError.

There is no public alloc feature yet. Allocation-backed APIs may move from std to a future alloc feature after a separate API review and test matrix update.