feat(exporter): add Markdown (.md) and PDF export support to pkg/exporter

[mendsec]

Feature Request

Description

catnet-io/engine currently provides export utilities (ExportJSON, ExportCSV, ExportXML) in pkg/exporter and pkg/export.

To support rich report generation in downstream applications (such as catnet-io/tui), pkg/exporter should be extended to support Markdown (.md) and PDF report generation.

Proposed Additions

• ExportMarkdown(report *results.ScanReport) ([]byte, error): Generates a structured Markdown document containing scan metadata, host discovery tables (IP, Hostname, MAC, OS/ports), and summary statistics.
• ExportPDF(report *results.ScanReport) ([]byte, error): Generates a clean PDF document version of the scan report.

Context

This feature request originates from catnet-io/tui feedback where users need native Markdown and PDF export options alongside JSON/CSV formats.