feat(ooxml): Drawing layer — images and shapes support

[arcaputo3]

Summary

Add support for reading, preserving, and writing drawing objects (images, shapes) in OOXML workbooks.

Motivation

Images and shapes are a common Excel feature. Currently xl has no drawing layer model — embedded images and shapes are silently dropped during round-trips.

Scope

• Parse xl/drawings/drawing*.xml and image relationships
• Domain model for Drawing (image, shape, position/anchor)
• Preserve drawings during read-write round-trips (surgical write)
• API for inserting images into sheets
• Support both one-cell and two-cell anchors

Non-goals (for now)

• SmartArt, 3D models
• Drawing editing/manipulation beyond insert

Migrated from Linear TJC-484.