Replace DOM parsing with a semantic pull-parser Markdown pipeline

[nilskruthoff]

• Replaced slide, relationship, and ODP DOM-style XML processing with event-driven pull parsing
• Added source-order and spatial reading-order strategies
• Added positioned bounds and source order to semantic blocks
• Added text roles for titles, subtitles, headings, body text, and captions
• Preserved individual paragraphs, alignment, runs, hyperlinks, and list metadata
• Added semantic tables with row spans, column spans, and covered-cell state
• Added image MIME type and alternative-text metadata
• Preserved mixed normal and list paragraphs in the same text box
• Preserved nested list levels and ordered-list start values
• Added strikethrough, superscript, subscript, hard-break, and field handling
• Rendered regular tables as GFM Markdown and merged tables as HTML
• Emitted standard Markdown image syntax with alternative text
• Added PresentationContainer::parse_document() for structured consumers

[codecov]

Codecov Report

❌ Patch coverage is 90.06496% with 260 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/parse_xml.rs	86.50%	156 Missing ⚠️
src/odp.rs	91.56%	65 Missing ⚠️
src/slide.rs	95.48%	18 Missing ⚠️
src/container.rs	67.74%	10 Missing ⚠️
src/xml.rs	91.89%	6 Missing ⚠️
src/metadata.rs	95.65%	4 Missing ⚠️
src/parse_rels.rs	96.96%	1 Missing ⚠️

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