Add multi-tax-id support and --output-table flag

[audy]

Summary

• --tax-id can now be specified multiple times to download assemblies from multiple taxa (OR logic)
• New --output-table flag writes a TSV with tax_id, lineage, accession, asm_name, organism, assembly_level, refseq_category, and path (use - for stdout)
• NCBIAssembly struct now includes assembly_accession, organism_name, and asm_name fields

Test plan

• cargo test passes
• gdl --tax-id 821 --tax-id 1350 --format fna --source refseq --dry-run returns assemblies from both taxa
• gdl --tax-id 821 --output-table out.tsv writes expected TSV columns
• gdl --tax-id 821 --output-table - streams TSV to stdout