confluence native format to markdown convertion golang package.

Scope lock:

• Focus only on parsing native Confluence content formats (body.storage, body.editor2, body.export_view) and converting them to correct Markdown.
• Do not implement Atlassian API integration, remote crawling, or authentication flows.

Markdown fidelity policy:

• Preserve semantics only when they are visible in markdown output.
• Do not emit HTML fallbacks for non-markdown-visible storage semantics.
• Examples of intentionally dropped output semantics: text color, text size, paragraph alignment, rich image layout attributes.

• Current state: docs/xml.md
• Roadmap: roadmap/xml.md
• Design: design/xml.md, design/rest.md
• Research: research/high.md
• Specs: specs/README.md, specs/xml.md

• Phases 1-6 completed: library skeleton, public contracts, typed validation errors, shared IR, normalization passes, deterministic markdown renderer, Storage XML parser, ADF parser, conversion orchestrator, fixture corpus, deterministic quality gates, and tests.

Canonical validation command:

• go test ./... -count=1