[ENH] Add sequence_statistics utility for RNA/DNA sequence analysis

[Vaishnav88sk]

Add gc_content(), nucleotide_composition(), and sequence_summary() utility functions for aptamer sequence profiling and screening.

These functions provide essential metrics (GC content, nucleotide breakdown, batch statistics) that researchers need during aptamer design workflows.

Reference Issues/PRs

Fixes #576

What does this implement/fix? Explain your changes.

Adds a new pyaptamer/utils/_sequence_stats.py module with three utility functions for aptamer sequence profiling:

• gc_content(sequence) — Computes GC content as a float (0.0–1.0). Case-insensitive, supports both DNA (ACGT) and RNA (ACGU).
• nucleotide_composition(sequence) — Returns a dict with per-nucleotide counts and frequencies. Unknown characters grouped under "other".
• sequence_summary(sequences) — Batch analysis returning a pd.DataFrame with columns: sequence, length, gc_content, A, C, G, T, U.
  All three are exported from pyaptamer.utils:

from pyaptamer.utils import gc_content, nucleotide_composition, sequence_summary

What should a reviewer concentrate their feedback on?

Did you add any tests for the change?

Yes - pyaptamer/utils/tests/test_sequence_stats.py with 15 tests covering:

• Normal DNA/RNA inputs, edge cases (empty string, single char)
• Case insensitivity
• Unknown character handling
• Type validation (TypeError for non-string inputs)
• Batch summary with variable-length sequences

Any other comments?

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

[Vaishnav88sk]

@fkiraly @NennoMP

[Vaishnav88sk]

@SimonBlanke