Clarify data validation philosophy: lenient import, strict export

[Copilot]

Addressed review feedback on PR #228 regarding direction_id validation in route patterns. The codebase intentionally accepts non-standard data during import and validates at export time.

Context

The bot reviewer suggested making direction_id required during import (using extract_required), but this conflicts with the design philosophy:

• Import: Accept lenient data, including nil/empty optional fields
• Export: Validate strictly against GTFS specification

Changes

No code changes required. Current implementation correctly allows nil values:

# row_parser.ex:74 - intentionally lenient
{:ok, direction_id} <- parse_direction_id(row_map["direction_id"])

# parse_direction_id/1 - allows nil/empty
def parse_direction_id(nil), do: {:ok, nil}
def parse_direction_id(""), do: {:ok, nil}

Schema validation via RoutePattern.changeset/2 enforces requirements when creating records for export, not during raw import.

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