docs: record that Replace-all cannot re-season an existing library

Measured rather than inferred: on a 116 episode library, disabling the
setting and running a full "Replace all metadata" refresh to completion
changed zero episode seasons and left all 30 playlist season names in
place.

This reconciles two apparently conflicting pieces of evidence.
MergeData does gate on `replaceData || !target.ParentIndexNumber
.HasValue`, which suggested a full replace would overwrite. But
MetadataService seeds the working item with the existing
ParentIndexNumber before any provider runs, so HasValue is never false
and the plugin is never asked to supply a replacement.

The important consequence was previously understated. The notes warned
only about disabling the setting, but stickiness is symmetric: enabling
it will not convert a library whose episodes are already imported into
year based seasons either. Only newly imported episodes, or a library
first scanned with the setting already on, group by playlist.

Adds a callout at the top of the enabling steps and documents the
conversion procedure: delete the Season entities and rescan, never the
Series.

Note: AI Generated Commit

RELEASE_NOTES.md

@@ -29,6 +29,10 @@ playlists are appended at the end and never renumber existing seasons.
 
 ### Enabling it
 
+> **Existing libraries:** episodes already imported into year-based seasons will **not** move.
+> `ParentIndexNumber` is sticky in Jellyfin — see "Turning the setting off" below for the measured
+> result and the conversion procedure. A brand-new library groups by playlist immediately.
+
 1. Replace `plugins/TubeArchivistMetadata/Jellyfin.Plugin.TubeArchivistMetadata.dll` with the new build.
 2. **Restart Jellyfin.** The Season metadata fetcher repair below runs at startup only.
 3. Check the log for `Enabled the TubeArchivist Season metadata fetcher on library <name>`.
@@ -97,9 +101,29 @@ season — but Jellyfin keeps the name it already has. A refresh alone will not
 The same applies to season *membership*: an episode's `ParentIndexNumber` is sticky, so a refresh
 cannot move episodes back into year-based seasons.
 
+**Measured, not assumed.** On a 116-episode library the setting was disabled and a full
+"Replace all metadata" refresh was run to completion. **Zero episodes changed season**, and all 30
+seasons kept their playlist names. This holds even though `MergeData` does gate on
+`replaceData || !target.ParentIndexNumber.HasValue`, because `MetadataService` seeds the working
+item with the *existing* `ParentIndexNumber` before any provider runs, so the value is never empty
+and the plugin is not asked to supply a new one.
+
 **To fully revert**, disable the setting and then remove the affected seasons so Jellyfin recreates
 them on the next scan.
 
+### The same limit applies when first enabling the feature
+
+This is the important consequence for existing installations. Because `ParentIndexNumber` is sticky
+in *both* directions, enabling the setting and refreshing will **not** move episodes that Jellyfin
+has already imported into year-based seasons. Those episodes keep their existing season.
+
+New episodes imported after enabling the setting are grouped by playlist correctly, and a library
+scanned for the first time with the setting already on is grouped entirely by playlist — verified on
+a brand-new library.
+
+To convert an existing year-grouped library, delete its **Season** entities (never the Series) and
+rescan.
+
 > ⚠️ **Do not delete the Series to reset it.** In Jellyfin, `DELETE /Items/{id}` on a Series, Season
 > or Episode **deletes the media files from disk**. There is no trash or recycle bin. Delete only the
 > Season entities, or point a throwaway library at a copy of the media.