Decode the compact one-byte element header

Zero length bits mean one data octet; zero padding and reserved IDs have separate meanings.

• The one-byte form MUST use outer profile 0xBEDE. Each element starts with four ID bits and four length bits. Valid IDs are 1–14. Length is data octets minus one: nibble 0 means 1 data octet and nibble 15 means 16. A zero-data-length element cannot be represented in this form (§4.2).
• The chosen header 0x30 means ID3 and length nibble 0; one data byte 7f follows. Header 0x3f instead means ID3 with 16 data bytes. The element header itself is not counted in its data length. No per-element four-byte alignment is required (§§4.1.2,4.2).
• Zero-valued bytes are padding, not ID 0 elements with one byte of data. Padding MAY appear between elements or at the end to reach the outer 32-bit boundary. A complete selected layout with data lengths 1/2/4, three element headers and two inter-element pad bytes uses 12 body octets: outer length 3 (§§4.1.2,4.2).
• ID 15 is reserved and MUST NOT be used. If encountered, ignore its length and terminate processing the entire extension; only earlier elements SHOULD be considered. Separately, an apparent ID 0 with a nonzero length field also requires termination with its length ignored (§§4.1.2,4.2). The helper keeps the earlier elements as its chosen disposition; it does not turn SHOULD into a mandatory universal discard/accept rule.
• Termination of element processing does not move the RTP payload boundary. The outer length still locates the end of the extension block; bytes after a reserved marker are not reinterpreted as media merely because element parsing stopped. Malformed body lengths stop the bounded helper before reading beyond the declared block; no new error packet is defined.