Function parse_nef 

pub fn parse_nef(bytes: &[u8]) -> Result<ParsedNef, String>

Parse a Neo N3 NEF3 file produced by build_nef_with_tokens.

Validates, in order:

• the 4-byte NEF3 magic,
• the trailing sha256(sha256(prefix))[..4] checksum,
• that every varint length prefix (source, token count, method name, script) is consistent with the remaining buffer,
• the single reserved byte (post-source) and two reserved bytes (pre-script) are all zero,
• method-token invariants matching those enforced by the builder (method.len() <= MAX_TOKEN_METHOD_LENGTH, !method.starts_with('_'), call_flags & !MAX_CALL_FLAGS == 0, tokens.len() <= MAX_METHOD_TOKENS).

Returns Err with a human-readable diagnostic on any violation.