Add decoded_cache HashMap, ensure_decoded(&mut self) for serial setup,
and sequence_bytes(&self) for parallel read access. This separates
mutation from borrowing so rayon workers can read decoded bytes without
external caching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Collapse 3 FASTA import methods (base, _force, _internal) into a single
add_sequence_collection_from_fasta() accepting a FastaImportOptions builder
with force() and namespaces() setters. Add extract_aliases_from_header()
for parsing namespaced aliases from FASTA headers during import.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

…s API to return Result

- Remove dead bbid variable in region_set error path
- Gate std::error::Error import behind http feature
- Fix CustomError to display its message instead of hardcoded string
- Add #[must_use] to FastaImportOptions builder methods
- Add clear_decoded_cache() for memory reclamation
- Change get_sequence_by_alias to return Result instead of Option
- Update Rust and Python tests for new API

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

…etter defrag than glibc- only load one seq in memory at a time properly