Add wrapped frontmatter support for non-Markdown files

[DandyLyons]

Summary

• add syntax-mapped wrapped YAML frontmatter parsing for supported non-Markdown text files
• integrate every md-utils fm leaf command with mapped selection, safe creation, and snapshot-checked atomic mutation
• document the syntax contract in command help, DocC, and user guides
• add shared black-box CLI test infrastructure and fixture-based parity coverage
• correct the unrelated OKF bundled-schema test so the complete suite is green

Verification

• swift test — 1,092 tests across 91 suites pass
• DocC generation for MarkdownUtilitiesCore and MarkdownUtilities passes with warnings as errors

Issue: #98