Stable id = path relative to memory/ without extension, with / between folders, e.g. 'entities/johnny'.
The store refuses an id without this form (see isWikiPageId) before it reads or writes a file.
One-line catalog summary.
ISO8601 last-update timestamp.
Provenance: ids of memory traces that fed this page.
Markdown body (frontmatter stripped).
Parsed [[wikilink]] targets found in the body.
A single wiki page: parsed frontmatter + markdown body.