fix: align markdown rootEntity override with promote identity

[Jurij89]

Summary

• fix issue documentIri pinning causes frontmatter rootEntity override to disagree with promote partitioning #122 by retargeting markdown import extraction to the resolved
  ootEntity when frontmatter overrides the default assertion-UAL identity
• keep _meta rows keyed by �ssertionUri while aligning data-graph rows 1/2/3 and content triples with the actual promotable entity subject
• update the mirrored import orchestration test harness and add an issue-focused regression test proving the override no longer stays informational-only

Verification

• pnpm exec vitest run test/extraction-markdown.test.ts
• pnpm exec vitest run test/import-file-integration.test.ts
• pnpm exec vitest run test/draft-lifecycle.test.ts
  Closes documentIri pinning causes frontmatter rootEntity override to disagree with promote partitioning #122.

[github-actions]

Codex review completed — no issues found.