Encode opaque bytes and counted ASCII strings

Count payload bytes, exclude padding, and respect the declared maximum.

• Opaque means uninterpreted bytes. Fixed opaque data declared opaque sample[5] carries exactly five bytes plus three residual zero bytes; it has no count prefix. The type description supplies its size (§4.9).
• Variable opaque data declared opaque sample<8> begins with an unsigned 32-bit byte count n. Five payload bytes therefore use count 00 00 00 05, five bytes and three zero bytes: twelve bytes total. The maximum8 is a declaration bound, not another transmitted number (§4.10).
• An XDR string is a counted sequence of ASCII bytes. The string "cat" has count 3, bytes63 61 74, then one zero pad byte. XDR does not append a C-style terminator as part of the string. A zero pad byte is outside the count; an embedded NUL can instead be a counted string byte and creates native-API hazards (§§4.11,8). No automatic UTF-8 extension is assumed.
• An omitted bound <> means a maximum of 2^32−1, not unlimited memory or a safe allocation size. Encoding a count beyond the declared maximum is an error. Decoders still need to ensure incoming sizes fit provisioned buffers even when the description has a large or omitted bound (§§4.10–4.11,8).
• The fourth state illustrates count 9 against an explicit bound 8 and stops a chosen decoder before allocation. This is a local safety gate, not a new wire exchange. Empty counted data has count 0 and no payload/padding. Residual bytes in the representation are zero; the helper additionally chooses strict rejection of nonzero padding, not an invented universal decoder MUST.