Function cborToBiguint

Convert CBOR to a non-negative bigint.

Matches Rust's TryFrom<CBOR> for BigUint.

Accepts:

• Major type 0 (unsigned integer)
• Tag 2 (positive bignum) with canonical byte string

Rejects:

• Major type 1 (negative integer) -> OutOfRange
• Tag 3 (negative bignum) -> OutOfRange
• Floating-point values -> WrongType
• Non-canonical bignum encodings -> NonCanonicalNumeric