perf: pre-compute column indices once per table; avoid Vec<char> in detect_columns

[Copilot]

Two hot-path allocations in the winget output parser eliminated — one per-row (repeated to_lowercase column lookups), one per-table (collecting header chars into a Vec<char>).

Changes

• detect_columns: replaced Vec<char> collect + index loop with char_indices().peekable(), giving (byte_offset, char) pairs directly with no heap allocation.

• PackageCols / SourceCols structs (#[derive(Copy, Clone)]): hold pre-computed Option<usize> column indices. Factory methods package_column_map() / source_column_map() run all find_column_ci calls and positional-fallback logic once per table instead of once per row.

// Before: find_column_ci called for every row (O(rows × cols) to_lowercase allocs)
fn parse_table_row(&self, line: &str, cols: &[(&str, usize)]) -> Option<Package> {
    let mut name_idx = Self::find_column_ci(cols, &["name", "nom", "nombre", "nome"]);
    let mut id_idx   = Self::find_column_ci(cols, &["id", "id."]);
    // ...
}

// After: indices computed once, passed as Copy struct
let col_map = Self::package_column_map(&col_positions);          // once per table
.filter_map(|line| self.parse_table_row(line, &col_positions, col_map))

No behaviour change — purely mechanical refactor of the parsing hot path.

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