Add DocxReader class that parses existing DOCX files back into
DocxDocument objects, enabling round-trip: read → modify → write.

Supports all paragraph styles, text formatting, lists with nesting,
tables with borders/colspan/rowspan, hyperlinks, bookmarks, page
breaks, and line breaks. Includes typed exceptions and graceful
degradation for unknown styles and missing optional parts.

New dependency: xml ^6.0.0 (pure Dart XML parser).
124 tests covering round-trip, unit parsers, edge cases.
Bumps version to 1.3.0.