Add shared export architecture and typed DOCX/PDF options

[lahiruudayakumara]

Parent: #14

Summary

Introduce shared export abstractions and typed configuration used by the new DOCX and PDF writers.

Scope

• Define common writer/exporter contracts that align with the existing Excel and CSV architecture.
• Add public TypeScript types for document metadata, page size, margins, orientation, headers, and footers.
• Define normalized intermediate data for workbook values, display values, styles, merged cells, and dimensions.
• Add validation with actionable errors for invalid or unsupported combinations.
• Preserve backward compatibility for existing exporters.

Acceptance criteria

• DOCX and PDF writers can consume the shared export model.
• Public options are strongly typed and exported from the package entry point.
• Defaults and validation behavior are documented in code.
• Existing Excel/CSV APIs and tests remain unchanged.