Problem: the detail buffer rendered metadata as read-only virtual text
overlays. Users could not edit status, priority, category, due, or
recurrence from the detail view.
Solution: render frontmatter as real `Key: value` text lines highlighted
via extmarks. On `:w`, `parse_detail_frontmatter()` validates every
field (status, priority bounds, `resolve_date`, `recur.validate`) and
aborts with `log.error()` on any invalid input. Removing a line clears
the field; editing the `# title` updates the description.
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