feat: precache resilience — circuit breaker, forensic diag log, safe version check

- Circuit breaker: abort preload after 5 consecutive network failures
  (success or 404 resets streak) instead of grinding doomed retries
- localStorage diag ring buffer (survives cache wipes): preload-start/
  done/abort, manifest-unavailable, version events; shown in Settings
- checkDataVersion: no longer wipes IDB on transient read errors
  (DataCache.has tri-state probe); logs old value on real migrations
- loadManifest: null result no longer cached forever (per-session fallback lock-in)

src/core/data-cache.ts

@@ -93,6 +93,24 @@ export class DataCache {
     }
   }
 
+  /**
+   * Tri-state presence probe: true/false when known, null on read error.
+   * Unlike get(), lets callers distinguish "absent" from "unreadable".
+   */
+  static async has(key: string): Promise<boolean | null> {
+    try {
+      const db = await getDB();
+      return await new Promise<boolean | null>((resolve) => {
+        const tx = db.transaction(STORE_NAME, 'readonly');
+        const request = tx.objectStore(STORE_NAME).count(IDBKeyRange.only(key));
+        request.onsuccess = () => resolve(request.result > 0);
+        request.onerror = () => resolve(null);
+      });
+    } catch {
+      return null;
+    }
+  }
+
   /**
    * Store a value in the cache.
    */

src/core/diag-log.ts

@@ -0,0 +1,50 @@
+/**
+ * Forensic event log — append-only ring buffer in localStorage.
+ *
+ * Deliberately stored OUTSIDE IndexedDB / CacheStorage so it survives
+ * DataCache.clear(), service-worker cache purges and version wipes.
+ * Used to diagnose transient offline/precache issues after the fact.
+ */
+
+const DIAG_KEY = 'ponomar-diag-log';
+const MAX_EVENTS = 30;
+
+export interface DiagEvent {
+  t: string;
+  event: string;
+  [key: string]: unknown;
+}
+
+export function diagLog(event: string, detail?: Record<string, unknown>) {
+  try {
+    const raw = localStorage.getItem(DIAG_KEY);
+    const arr: DiagEvent[] = raw ? JSON.parse(raw) : [];
+    arr.push({ t: new Date().toISOString(), event, ...(detail ?? {}) });
+    while (arr.length > MAX_EVENTS) arr.shift();
+    localStorage.setItem(DIAG_KEY, JSON.stringify(arr));
+  } catch { /* best-effort */ }
+}
+
+export function getDiagLog(): DiagEvent[] {
+  try {
+    const raw = localStorage.getItem(DIAG_KEY);
+    const arr = raw ? JSON.parse(raw) : [];
+    return Array.isArray(arr) ? arr : [];
+  } catch {
+    return [];
+  }
+}
+
+function formatEvent(e: DiagEvent): string {
+  const time = (e.t || '').slice(11, 19);
+  const parts: string[] = [];
+  for (const [k, v] of Object.entries(e)) {
+    if (k === 't' || k === 'event') continue;
+    parts.push(`${k}=${String(v)}`);
+  }
+  return `${time} ${e.event}${parts.length ? ' ' + parts.join(' ') : ''}`;
+}
+
+export function diagTail(): string {
+  return getDiagLog().map(formatEvent).join('\n');
+}

src/core/i18n.ts

@@ -515,6 +515,7 @@ export interface Translations {
     offlineMenaion: string;
     offlineBible: string;
     offlinePreload: string;
+    offlineAborted: string;
     offlineClearCache: string;
     offlineCalculating: string;
     offlineDone: string;
@@ -1084,6 +1085,7 @@ const translations: Record<LanguageCode, Translations> = {
       offlineMenaion: 'Menaion',
       offlineBible: 'Bible',
       offlinePreload: 'Preload',
+      offlineAborted: 'Preloading stopped: network unavailable. Check your connection and try again.',
       offlineClearCache: 'Clear cache',
       offlineCalculating: 'Calculating storage...',
       offlineDone: 'Done! Data cached for offline use.',
@@ -1651,6 +1653,7 @@ const translations: Record<LanguageCode, Translations> = {
       offlineMenaion: 'Минея',
       offlineBible: 'Библия',
       offlinePreload: 'Загрузить',
+      offlineAborted: 'Загрузка прервана: сеть недоступна. Проверьте соединение и повторите попытку.',
       offlineClearCache: 'Очистить кэш',
       offlineCalculating: 'Расчёт размера...',
       offlineDone: 'Готово! Данные закэшированы для офлайн-использования.',
@@ -2218,6 +2221,7 @@ const translations: Record<LanguageCode, Translations> = {
       offlineMenaion: 'Минїа',
       offlineBible: 'Би́блїа',
       offlinePreload: 'Загрузити',
+      offlineAborted: 'Загрузка прервана: сеть недостꙋпна. Про̀верьте соедине́нiе и҆ повтори́те попы́ткꙋ.',
       offlineClearCache: 'Очистити кешъ',
       offlineCalculating: 'Расчётъ размѣра...',
       offlineDone: 'Готово! Данныя закешированы для офлайн-потребленїя.',

src/core/offline-manager.ts

@@ -15,6 +15,7 @@
  */
 
 import { DataCache } from './data-cache';
+import { diagLog } from './diag-log';
 
 export interface PreloadOptions {
   languages: string[];
@@ -29,6 +30,7 @@ export interface PreloadProgress {
   done: boolean;
   failed: number;
   failedFiles?: string[];
+  aborted?: boolean;
 }
 
 export interface CacheStats {
@@ -60,7 +62,14 @@ function loadManifest(): Promise<Record<string, Record<string, string[]>> | null
       } catch {
         return null;
       }
-    })();
+    })().then((m) => {
+      // Don't cache a failure forever — allow retry on next call
+      if (m === null) {
+        diagLog('manifest-unavailable');
+        manifestPromise = null;
+      }
+      return m;
+    });
   }
   return manifestPromise;
 }
@@ -112,6 +121,7 @@ export class OfflineManager {
    * Returns a PreloadProgress object that can be polled for progress.
    */
   static async preload(options: PreloadOptions): Promise<PreloadProgress> {
+    const startedAt = Date.now();
     const filesToFetch: string[] = [];
     const m = await loadManifest();
 
@@ -149,7 +159,16 @@ export class OfflineManager {
     };
     OfflineManager._progress = progress;
 
+    diagLog('preload-start', {
+      langs: options.languages.join(','),
+      types: options.types.join(','),
+      bibles: (options.bibleTranslations || []).length,
+      total: progress.total,
+      manifest: m !== null,
+    });
+
     // Fetch files sequentially to avoid overwhelming the network
+    let netFailStreak = 0;
     for (const file of uniqueFiles) {
       progress.current++;
       progress.file = file;
@@ -186,12 +205,28 @@ export class OfflineManager {
       }
       if (!success && !notFound) {
         progress.failed++;
+        netFailStreak++;
         if (!progress.failedFiles) progress.failedFiles = [];
         if (progress.failedFiles.length < 10) progress.failedFiles.push(file);
+        if (netFailStreak >= 5) {
+          // Circuit breaker — network appears dead; abort instead of grinding
+          progress.aborted = true;
+          diagLog('preload-abort', { failed: progress.failed, current: progress.current });
+          break;
+        }
+      } else {
+        // Success or 404 both prove connectivity
+        netFailStreak = 0;
       }
     }
 
     progress.done = true;
+    diagLog('preload-done', {
+      total: progress.total,
+      failed: progress.failed,
+      ms: Date.now() - startedAt,
+      aborted: progress.aborted === true,
+    });
     OfflineManager._progress = null;
     return progress;
   }

src/ui/app.ts

@@ -11,6 +11,7 @@ import { JDate } from '../core/jdate';
 import { loadSettings, type Settings } from './settings-view';
 import { getTranslations, type LanguageCode } from '../core/i18n';
 import { DataCache } from '../core/data-cache';
+import { diagLog } from '../core/diag-log';
 
 type View = 'calendar' | 'bible' | 'prayer' | 'akathists' | 'parimii' | 'horologion' | 'sbornik' | 'paraclete' | 'irmologion' | 'menaion' | 'triodion' | 'lives' | 'settings';
 
@@ -75,13 +76,25 @@ export class App {
   private async checkDataVersion() {
     const DATA_VERSION = '2026-08-04'; // bump when data structure changes
     try {
+      const present = await DataCache.has('__data_version__');
+      if (present === null) {
+        // Unreadable IDB — never wipe the cache on a flaky read
+        diagLog('version-check', { result: 'read-error', action: 'skip' });
+        return;
+      }
+      if (!present) {
+        await DataCache.set('__data_version__', DATA_VERSION);
+        diagLog('version-init');
+        return;
+      }
       const cachedVer = await DataCache.get('__data_version__') as string | null;
       if (cachedVer !== DATA_VERSION) {
         await DataCache.clear();
         await DataCache.set('__data_version__', DATA_VERSION);
+        diagLog('version-clear', { old: cachedVer });
       }
-    } catch {
+    } catch (err) {
-      // Silently fail — cache is best-effort
+      diagLog('idb-error', { where: 'checkDataVersion', msg: String(err) });
     }
   }
 

src/ui/settings-view.ts

@@ -5,6 +5,7 @@
 import { getTranslations, type LanguageCode } from '../core/i18n';
 import { OfflineManager } from '../core/offline-manager';
 import { DataCache } from '../core/data-cache';
+import { diagTail } from '../core/diag-log';
 
 const LANGUAGES = [
   { code: 'en', name: 'English', local: 'English' },
@@ -303,6 +304,7 @@ export class SettingsView {
             </div>
             <p id="offline-progress-text" class="text-xs text-navy-light">0 / 0 ${t.settings.offlineDataTypes}</p>
           </div>
+          <pre id="diag-log" class="text-[10px] font-mono text-navy-light whitespace-pre-wrap max-h-32 overflow-auto mt-1"></pre>
         </div>
 
         <!-- About -->
@@ -552,13 +554,17 @@ export class SettingsView {
       }).then((prog) => {
         if (progressBar) progressBar.style.width = '100%';
         if (progressText) {
-          if (prog.failed > 0) {
+          if (prog.aborted) {
+            progressText.textContent = t.settings.offlineAborted;
+          } else if (prog.failed > 0) {
             const failList = prog.failedFiles?.length ? `: ${prog.failedFiles.join(', ')}` : '';
             progressText.textContent = `${t.settings.offlineDone} ${prog.failed} ${t.settings.offlineFailed}${failList}`;
           } else {
             progressText.textContent = t.settings.offlineDone;
           }
         }
+        const diagEl = document.getElementById('diag-log');
+        if (diagEl) diagEl.textContent = diagTail();
         OfflineManager.getStats().then(stats => {
           const statsEl = document.getElementById('offline-stats');
           if (statsEl) {
@@ -601,6 +607,9 @@ export class SettingsView {
       }
     });
 
+    const diagEl = document.getElementById('diag-log');
+    if (diagEl) diagEl.textContent = diagTail();
+
     // Check cache status for each data type and set checkbox states
     (async () => {
       const checkCache = async (key: string): Promise<boolean> => {
@@ -678,10 +687,14 @@ export class SettingsView {
           if (progressBar) progressBar.style.width = '100%';
           if (progressText) {
             const failList = prog.failedFiles?.length ? `: ${prog.failedFiles.join(', ')}` : '';
-            progressText.textContent = prog.failed > 0
+            progressText.textContent = prog.aborted
-              ? `${t.settings.offlineDone} ${prog.failed} ${t.settings.offlineFailed}${failList}`
+              ? t.settings.offlineAborted
-              : t.settings.offlineDone;
+              : prog.failed > 0
+                ? `${t.settings.offlineDone} ${prog.failed} ${t.settings.offlineFailed}${failList}`
+                : t.settings.offlineDone;
           }
+          const diagEl = document.getElementById('diag-log');
+          if (diagEl) diagEl.textContent = diagTail();
           OfflineManager.getStats().then(stats => {
             const statsEl = document.getElementById('offline-stats');
             if (statsEl) {