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/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,