fix(codecs): correct wavelength padding key in pad_spectrum

[hopeful0]

While using the SDSS spectrum codec, I kept seeing failures consistent with #61 even after #64 landed. Tracing the encode path showed that pad_spectrum (introduced in #64) pads flux, ivar, mask, and wavelength, but padding_values still used the key "lambda". Because the modality field is wavelength, those padded positions were filled with 0 instead of the intended 99999 sentinel.

This change updates the dict key to "wavelength" so padded wavelength bins match the other fields.

[hopeful0]

By the way, there's a question: according to pad_spectrum, should we set the wavelength of all invalid data (mask=True) to 99999 instead of 0 or other values?