fix: ignore frontmatter examples in fenced code blocks

[Iktahana]

Summary:

• make the late-frontmatter guard track CommonMark backtick and tilde fences
• require matching fence characters and sufficient closing-fence length
• keep genuine late frontmatter outside code fences on the literal recovery path
• cover the production docs shape plus indentation and invalid-fence edge cases

Root cause:
The guard around markdown-rs scanned raw delimiter lines without tracking fenced code blocks. MDI/YAML examples in the Full syntax reference were therefore mistaken for a second frontmatter block, causing the entire page to use literal fallback.

Local verification:

• cargo fmt --check
• cargo clippy with warnings denied
• cargo test: 77 unit tests plus all integration and doc tests pass; environment-dependent native Chromium smoke excluded after it failed to launch locally
• pnpm docs:build: 467 pages
• generated syntax reference contains rendered strong and link elements, with no literal prerequisite Markdown

remarkMdi and Rust-backed documentation parsing remain enabled.

[codecov]

Codecov Report

❌ Patch coverage is 98.86364% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mdi-core/src/lib.rs	98.86%	1 Missing ⚠️

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