docs: define Phase 3 independent library program

[ZSeanYves]

Change summary

Define Phase 3 as five separately initiated subphases: safezip, xml-stream, opc, pdf-extract, and the HTML/YAML/TOML shadow plus Markdown conformance evaluation.

Scope

Documentation-only update to docs/project-maintenance-plan.md. No library repository or implementation is created in this PR.

Risk and ownership

R0 documentation change. It establishes future R3 library, security, dependency, and release gates without changing current product behavior.

Contract and compatibility

Phase 0-2 behavior remains unchanged. Independent libraries must not depend on MarkItDown types; MarkItDown may consume only released versions through thin adapters and dual-run migration.

Verification

• python3 tools/governance/check_documentation.py
• python3 -m unittest tools/governance/tests/test_governance.py
• git diff --check

Checklist

• Five Phase 3 subphases have explicit scope, deliverables, dependency order, exit gates, and rollback boundaries.
• Independent repository and release requirements are explicit.
• No implementation code or generated interface changed.
• Generated artifacts and regeneration command: none; documentation-only change.