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/App.tsx

@@ -10,6 +10,7 @@ import {
   navigateToSession as navigateToSessionState,
   openAnimeEpisodeDetail,
   openOverviewMediaDetail,
+  openSessionsMediaDetail,
   switchTab,
 } from './lib/stats-navigation';
 
@@ -110,6 +111,10 @@ export function App() {
     [],
   );
 
+  const navigateToSessionsMediaDetail = useCallback((videoId: number) => {
+    setViewState((prev) => openSessionsMediaDetail(prev, videoId));
+  }, []);
+
   const openWordDetail = useCallback((wordId: number) => {
     setGlobalWordId(wordId);
   }, []);
@@ -155,7 +160,14 @@ export function App() {
               }
               onBack={() => setViewState((prev) => closeMediaDetail(prev))}
               backLabel={
-                mediaDetail.origin.type === 'overview' ? 'Back to Overview' : 'Back to Library'
+                mediaDetail.origin.type === 'overview'
+                  ? 'Back to Overview'
+                  : mediaDetail.origin.type === 'sessions'
+                    ? 'Back to Sessions'
+                    : 'Back to Library'
+              }
+              onNavigateToAnime={
+                mediaDetail.origin.type === 'anime' ? undefined : navigateToAnime
               }
             />
           </Suspense>
@@ -242,6 +254,7 @@ export function App() {
                     onClearInitialSession={() =>
                       setViewState((prev) => ({ ...prev, focusedSessionId: null }))
                     }
+                    onNavigateToMediaDetail={navigateToSessionsMediaDetail}
                   />
                 </Suspense>
               </section>

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}

stats/src/components/sessions/SessionsTab.tsx

@@ -26,9 +26,14 @@ function groupSessionsByDay(sessions: SessionSummary[]): Map<string, SessionSumm
 interface SessionsTabProps {
   initialSessionId?: number | null;
   onClearInitialSession?: () => void;
+  onNavigateToMediaDetail?: (videoId: number) => void;
 }
 
-export function SessionsTab({ initialSessionId, onClearInitialSession }: SessionsTabProps = {}) {
+export function SessionsTab({
+  initialSessionId,
+  onClearInitialSession,
+  onNavigateToMediaDetail,
+}: SessionsTabProps = {}) {
   const { sessions, loading, error } = useSessions();
   const [expandedId, setExpandedId] = useState<number | null>(null);
   const [search, setSearch] = useState('');
@@ -125,6 +130,7 @@ export function SessionsTab({ initialSessionId, onClearInitialSession }: Session
                     onToggle={() => setExpandedId(expandedId === s.sessionId ? null : s.sessionId)}
                     onDelete={() => void handleDeleteSession(s)}
                     deleteDisabled={deletingSessionId === s.sessionId}
+                    onNavigateToMediaDetail={onNavigateToMediaDetail}
                   />
                   {expandedId === s.sessionId && (
                     <div id={detailsId}>

stats/src/lib/stats-navigation.ts

@@ -1,7 +1,10 @@
 import type { SessionSummary } from '../types/stats';
 import type { TabId } from '../components/layout/TabBar';
 
-export type MediaDetailOrigin = { type: 'anime'; animeId: number } | { type: 'overview' };
+export type MediaDetailOrigin =
+  | { type: 'anime'; animeId: number }
+  | { type: 'overview' }
+  | { type: 'sessions' };
 
 export interface MediaDetailState {
   videoId: number;
@@ -92,6 +95,24 @@ export function openOverviewMediaDetail(
   };
 }
 
+export function openSessionsMediaDetail(
+  state: StatsViewState,
+  videoId: number,
+): StatsViewState {
+  return {
+    activeTab: 'sessions',
+    selectedAnimeId: null,
+    focusedSessionId: null,
+    mediaDetail: {
+      videoId,
+      initialSessionId: null,
+      origin: {
+        type: 'sessions',
+      },
+    },
+  };
+}
+
 export function closeMediaDetail(state: StatsViewState): StatsViewState {
   if (!state.mediaDetail) {
     return state;
@@ -106,6 +127,15 @@ export function closeMediaDetail(state: StatsViewState): StatsViewState {
     };
   }
 
+  if (state.mediaDetail.origin.type === 'sessions') {
+    return {
+      activeTab: 'sessions',
+      selectedAnimeId: null,
+      focusedSessionId: null,
+      mediaDetail: null,
+    };
+  }
+
   return {
     activeTab: 'anime',
     selectedAnimeId: state.mediaDetail.origin.animeId,