fix(stats): preserve pruned first_seen and stamp seen_ms at observation

- Record occurrence seen_ms from the observation time instead of flush time
- On session delete, only move first_seen when removed timestamps prove it was deleted; keep earlier dates from retention-pruned or legacy history
- Exact refresh fallback keeps subtraction-exact frequency and preserved first_seen
- Add tests for pruned history, legacy rows, and delayed flushes; update docs and change fragments

docs-site/immersion-tracking.md

@@ -124,4 +124,6 @@ Leave out `--lookback-days` to scan all history. Word and kanji counts are corre
 
 By default SubMiner keeps everything. To limit history, set `immersionTracking.retentionPreset` to `minimal`, `balanced`, or `deep-history`, or set the `immersionTracking.retention.*Days` values yourself (`0` keeps all). Lifetime totals and vocabulary counts are stored separately and stay exact when old sessions are pruned.
 
+Deleting retained sessions preserves earlier word and kanji discovery dates from pruned history. New occurrences record observation time so deletion can update their dates exactly. Older history used write time, so an earlier discovery date is preserved when its origin cannot be determined.
+
 See [Immersion tracking](/configuration#immersion-tracking) and [Stats dashboard](/configuration#stats-dashboard) in the config reference for every option and default.