fix: broaden markdown extractor IRI validation

[Jurij89]

Summary

• replace the remaining narrow http/https/did/urn IRI allowlists in markdown-extractor.ts with a shared absolute-scheme + isSafeIri helper
• preserve valid non-whitelist IRIs like tag:, doi:, and ark: in frontmatter type, generic frontmatter scalar values, and Dataview field values
• stop passing malformed scheme-prefixed values through as raw IRIs by falling back to the existing schema-class or literal branches
• add focused extractor regressions for the three issue-123 sites

Verification

• pnpm exec vitest run test/extraction-markdown.test.ts
• pnpm exec vitest run test/import-file-integration.test.ts

Docs

• No separate dkgv10-spec PR needed for this fix. The current spec already describes these values generically and does not define the narrow implementation allowlist that this PR removes.