TSV output formatter

[crashfrog]

Parent

#58 (Phase 1 MVP: Evidence-Informed Alignment with Deferred Filtering)

What to build

Implement TSV formatter in phraya-filter. Converts filtered VariantObservations to tab-separated values with user-selectable columns. Includes header row. Handles escaping of special characters (tabs, newlines in provenance fields). Simpler than VCF, useful for R/Python analysis.

Acceptance criteria

• format_tsv(observations: Iterator, columns: &[Column]) → String
• Column enum: Position, RefBase, AltBases, Coverage, Mapq, Cigar, EditDistance, Confidence, Provenance
• Default columns: Position, RefBase, AltBases, Coverage, Mapq
• Header row with column names
• Escape special characters: tabs (\t → \t), newlines (\n → \n)
• Tests: single observation → TSV row with correct columns
• Tests: column selection (only Position + Coverage) → only those columns in output
• Tests: special characters in provenance → escaped correctly
• Tests: multi-allelic site → ALT column contains comma-separated alleles

Blocked by

#81 (filter library)

[crashfrog]

Resolved by #110