Duplicate frontmatter keys: check the mapping, not the lines

Follow-on from #164, which [#239](https://github.com/dmarx/luria/pull/239) closed.

## Prior work

#239 shipped `luria/frontmatter_shape.py`, which inspects the raw `---` block before `yaml.safe_load` and reports two things: a line opening at column 0 with `

Start with luria/frontmatter_shape.py and inspect how check_frontmatter and check_journals process frontmatter. Replace the raw line scan with duplicate-key detection through the YAML loader, then verify that nested and differently quoted duplicates are caught and that journal entries receive the same shape checks.

Written by the indexing model from the issue text.