feat(ooxml): Type-safe chart model

[arcaputo3]

Summary

Add a type-safe domain model for Excel charts with read/write support.

Motivation

Charts are a major Excel capability with no representation in xl today. Chart XML is preserved as opaque blobs during round-trips but cannot be inspected, created, or modified programmatically.

Scope

• Parse xl/charts/chart*.xml into typed AST
• Domain model: Chart, Series, Axis, Legend, ChartType (bar, line, pie, scatter, etc.)
• Preserve charts during round-trips
• API for creating simple charts from sheet data ranges
• CLI: xl chart command for basic chart creation

Non-goals (for now)

• Pivot charts
• Chart templates
• Advanced chart customization (3D, sparklines)

Migrated from Linear TJC-318.