feat(stats): add media-detail navigation from Sessions rows; fix(tokenizer): exclude そうだ auxiliary-stem from annotations

- Added hover-revealed ↗ button on SessionRow that navigates to the
  anime media-detail view for the session's videoId
- Added `sessions` origin type to MediaDetailOrigin and
  openSessionsMediaDetail() / closeMediaDetail() handling so the
  back button returns correctly to the Sessions tab ("Back to Sessions")
- Wired onNavigateToMediaDetail down through SessionsTab → SessionRow
- Excluded tokens with MeCab POS3 = 助動詞語幹 (e.g. そうだ grammar tails)
  from subtitle annotation metadata so frequency, JLPT, and N+1 styling
  no longer apply to grammar-tail tokens
- Added annotation-stage unit test and end-to-end tokenizeSubtitle test
  for the そうだ exclusion path
- Updated docs-site changelog, immersion-tracking, and
  subtitle-annotations pages to reflect both changes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

stats/src/components/sessions/SessionRow.tsx

@@ -11,6 +11,7 @@ interface SessionRowProps {
   onToggle: () => void;
   onDelete: () => void;
   deleteDisabled?: boolean;
+  onNavigateToMediaDetail?: (videoId: number) => void;
 }
 
 function CoverThumbnail({
@@ -56,6 +57,7 @@ export function SessionRow({
   onToggle,
   onDelete,
   deleteDisabled = false,
+  onNavigateToMediaDetail,
 }: SessionRowProps) {
   const displayWordCount = getSessionDisplayWordCount(session);
   const knownWordsSeen = session.knownWordsSeen;
@@ -109,6 +111,20 @@ export function SessionRow({
           {'\u25B8'}
         </div>
       </button>
+      {onNavigateToMediaDetail != null && session.videoId != null ? (
+        <button
+          type="button"
+          onClick={(e) => {
+            e.stopPropagation();
+            onNavigateToMediaDetail(session.videoId!);
+          }}
+          aria-label={`View overview for ${session.canonicalTitle ?? 'Unknown Media'}`}
+          className="absolute right-10 top-1/2 -translate-y-1/2 w-5 h-5 rounded border border-ctp-surface2 text-transparent hover:border-ctp-blue/50 hover:text-ctp-blue hover:bg-ctp-blue/10 transition-colors opacity-0 group-hover:opacity-100 focus:opacity-100 flex items-center justify-center"
+          title="View anime overview"
+        >
+          {'\u2197'}
+        </button>
+      ) : null}
       <button
         type="button"
         onClick={onDelete}