fix(gtfs): accept orphan calendar service rules

[adot-7]

Closes #91

Summary

• accept valid calendar and calendar_dates service definitions that are unused by trips
• retain reverse validation for trip service IDs against the supplied calendar definition set
• add deterministic Load + BuildIndexes regression coverage for orphan calendar rules

Tests

• gofmt
• go test ./...
• go vet ./...
• go build ./...
• git diff --check
• read-only user feed Load + BuildIndexes validation passed

The user-provided feed under mapdata/extracted was accessed read-only and no mapdata files are included.