[TEST] #84: TSV output formatter

[crashfrog]

Summary

Added RED phase acceptance tests for TSV output formatter in phraya-filter crate. Tests define the API surface and acceptance criteria without implementing the feature.

API Surface Assumed by These Tests

// Column enum for TSV output selection
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Column {
    Position,      // 0-indexed reference position
    RefBase,       // Reference base as char
    AltBases,      // Comma-separated alt alleles, sorted, "." if none
    Coverage,      // From local_coverage[0], "0" if empty
    Mapq,          // Mapping quality
    Cigar,         // CIGAR string
    EditDistance,  // Edit distance
    Confidence,    // Confidence score (0.0-1.0)
    Provenance,    // Provenance with escaped special chars
}

// Format with specified columns
pub fn format_tsv(
    observations: impl Iterator<Item = VariantObservation>,
    columns: &[Column],
) -> String;

// Format with default columns: [Position, RefBase, AltBases, Coverage, Mapq]
pub fn format_tsv_with_defaults(
    observations: impl Iterator<Item = VariantObservation>,
) -> String;

Test Coverage

• Header row: tab-separated column names
• Data rows: tab-separated values matching column order
• Column selection: any subset of Column enum
• Default columns: Position, RefBase, AltBases, Coverage, Mapq
• Special character escaping: \t → \t, \n → \n in provenance fields
• Multi-allelic handling: sorted, comma-separated alt alleles
• Empty cases: zero coverage, no alt alleles (shows "."), empty observations
• All column types: tested individually and in combination

Filter Command

Run tests with: cargo test --lib tsv

All 15 tests currently fail at RED phase (functions not implemented).

🤖 Generated with Claude Code

[crashfrog]

Closing stale test PR - implementation PR #110 has been merged.