Function encode_index_key 

pub fn encode_index_key(
    spec: &IndexSpec,
    fields: &[Dynamic],
) -> Result<EncodedIndexKey>

Encode fields into a single index key under the kind-specific rules.

• Standard, Unique, Each: fields.len() must be 1; the output is the order-preserving encoding of that field, with no envelope tag. The caller appends the Id suffix for non-unique kinds.
• Composite: fields.len() must equal spec.key_paths.len() (≥ 2 by IndexSpec::validate); the output is the composite-envelope tag (0x80) followed by the concatenated per-field encodings.

Errors

• Error::InvalidArgument if fields.len() disagrees with the kind’s per-spec contract.
• Error::Codec if a String field contains an embedded 0x00 byte.