Calculate residue counts, dinucleotide frequencies, GC content, and grouped totals for DNA sequences.
Accepts FASTA format (lines starting with > are headers) or raw sequences. Non-ATGCN characters are stripped. Dinucleotide frequencies use a sliding window (overlapping pairs). GC% = (G+C)/total × 100. Purines = A+G; Pyrimidines = C+T.