P4: Punctuation/formatting-only headings

[RE-codes]

Story

As a vault user with an unusual heading like ## --- or ## **, I want the plugin to either resolve it consistently with GitHub or document the limitation. Rare in practice.

Tasks

• Add slug unit tests for: ## --- (expected slug per github-slugger), ## ** (expected: empty after strip → likely a degenerate slug), ## . (punctuation only).
• Confirm github-slugger output and document any edge cases in README.
• If degenerate slugs cause crashes, fix; otherwise accept as documented.

Acceptance Criteria

• Slug tests pass.
• README mentions degenerate-heading behavior briefly.
• Matrix P4 row updated.

References

• Parent epic: Epic: P4 Lower-Priority Readiness #93