test(02-01): add extractFrontmatter and reconstructFrontmatter unit tests
- extractFrontmatter: simple key-value, quotes, nested objects, block arrays,
inline arrays, REG-04 quoted comma edge case, empty/missing frontmatter,
emoji/non-ASCII, object-to-array conversion, empty line skipping
- reconstructFrontmatter: simple values, empty/short/long arrays, quoted
colons/hashes, nested objects, nested arrays, null/undefined skipping
- Round-trip identity tests: simple, nested, and multi-type fixtures