Expand description
Format-drift diagnostics: scan a session file and report any entry types or content-item types the typed parsers don’t recognize.
Used when the --debug-unknowns CLI flag is set. The cost is one extra
serde_json::Value parse per line — only runs with the flag.
Output is intentionally terse and machine-greppable: one section per format, sorted alphabetically by tag, with the first three line numbers where each unknown was seen.