[antonreshetov]

Storage mutated runtime entities before writing the file. If the write failed (EACCES, ENOSPC, etc.), the cache kept the unsaved change: GET, UI and MCP revisions showed it as saved, and the edit was lost on rescan or restart.

• Add persistOrRestore helper: snapshots the fields changed before the write and restores them if the write throws, then rethrows.
• Apply it to note body content, note properties, tag add/remove (notes and snippets), and snippet fragment update/create/delete.
• saveState and search index updates stay outside the rollback, since the file is already written at that point.
• MCP update_note / update_snippet: a content-only write failure now returns STORAGE_ERROR instead of PARTIAL_UPDATE, and the revision stays unchanged. After a partial update, the returned revision now matches the disk.

updateNote / updateSnippet (rename/move) are not covered: they move files and update the state index before writing, so a field rollback is not enough there.