docs(notifications): add Android plugin handout for empty-fetch dual schedule

Document PLUGIN_NOTIFICATION_FIX_ANDROID diagnosis and recommended changes in
the daily-notification-plugin repo, verified against plugin 3.0.0.

doc/PLUGIN_NOTIFICATION_FIX_ANDROID.md

@@ -0,0 +1,129 @@
+# Android plugin: New Activity notification when API has no activities
+
+**Audience:** Maintainers of `@timesafari/daily-notification-plugin` (Android / Kotlin).  
+**Host app:** TimeSafari (`crowd-funder-for-time-pwa`) — this file lives in the **app** repo only as a handoff; apply changes in the **plugin** repo.
+
+**Problem (product):** “New Activity” should notify only when the API reports new/updated activity. The host’s native fetcher (`TimeSafariNativeFetcher`) returns an **empty** `List<NotificationContent>` when the API’s `data` array is empty. Users still see a **daily** local notification.
+
+**Version note:** This diagnosis was first written against older plugin builds (e.g. **2.1.x / 2.2.x**). After upgrading the host to **`@timesafari/daily-notification-plugin` 3.0.0**, the Android files below were **re-read** from `node_modules`. The relevant logic is **unchanged** in 3.0.0: the same two mechanisms still explain unwanted daily notifications when the API returns no rows. If you maintain the plugin, re-verify after each major release.
+
+**Root cause (Android, confirmed in plugin v3.0.0 sources under `node_modules`):** Two mechanisms interact:
+
+1. **`FetchWorker.kt` — empty native fetch is converted to synthetic JSON instead of “skip”**  
+   When the dual prefetch runs with the native fetcher and the list is empty, `notificationContentsToDualPayloadBytes` **replaces** the empty list with a JSON payload `"No updates"` / `"No new content"`, and the work unit still completes successfully. The dual path then **always** arms the chained notify alarm when `isDual && nextNotifyAt > 0L` — so a notification is still scheduled for the notify window.
+
+   Reference (plugin):
+
+   ```kotlin
+   // FetchWorker.kt — notificationContentsToDualPayloadBytes (~371–374 in v3.0.0)
+   if (contents.isEmpty()) {
+       return """{"title":"No updates","body":"No new content"}""".toByteArray(Charsets.UTF_8)
+   }
+   ```
+
+   ```kotlin
+   // FetchWorker.kt — doWork(), tail of success path (~306–309 in v3.0.0)
+   if (isDual && nextNotifyAt > 0L) {
+       DualScheduleNotifyScheduler.scheduleChainedNotifyAlarm(applicationContext, nextNotifyAt)
+       DualScheduleFetchRecovery.enqueueFromPersistedConfig(applicationContext)
+   }
+   ```
+
+2. **`DualScheduleHelper.kt` — `fallbackBehavior: "show_default"` uses `userNotification` defaults**  
+   At display time, if there is **no** fresh dual-scope cache within `relationship.contentTimeout`, the helper falls back to the **persisted** `userNotification.title` / `userNotification.body` when `fallbackBehavior` is `"show_default"`. The host app sets those defaults to copy such as “New Activity” / “Check your starred projects…”, so the user sees that **even when the API had nothing**, if the cache path doesn’t supply something else.
+
+   Reference (plugin):
+
+   ```kotlin
+   // DualScheduleHelper.kt — resolveDualContentBlocking (simplified; ~31–57 in v3.0.0)
+   val fallbackBehavior = relationship?.optString("fallbackBehavior", "show_default") ?: "show_default"
+   val defaultTitle = userNotification.optString("title", "Daily Notification")
+   val defaultBody = userNotification.optString("body", "Your daily update is ready")
+   // ...
+   } else {
+       if (fallbackBehavior != "show_default") return null
+       Pair(defaultTitle, defaultBody)
+   }
+   ```
+
+**TypeScript contract (plugin `src/definitions.ts` in v3.0.0 — `DualScheduleConfiguration.relationship`):**
+
+```ts
+relationship?: {
+  autoLink: boolean;
+  contentTimeout: number;
+  fallbackBehavior: 'skip' | 'show_default' | 'retry';
+};
+```
+
+`skip` is only partially useful on Android **with the current fetch implementation**: it avoids the **default title/body** branch in `DualScheduleHelper` when cache is missing/stale, but it does **not** by itself stop a notification if the fetch path still materializes content (including the synthetic `"No updates"` payload) or if chained notify is already armed.
+
+**3.0.0 vs 2.2.x:** Plugin **3.0.0** advertises broader features (e.g. TTL-at-fire, observability). Those do **not** replace the dual-fetch pipeline inspected here: `FetchWorker` still maps an empty native list to JSON and still schedules the chained notify on success; `DualScheduleHelper` still applies `show_default` vs defaults when cache is absent or outside `contentTimeout`. Revisit this doc if a future release changes `notificationContentsToDualPayloadBytes` or the dual notify gate.
+
+---
+
+## Recommended plugin changes (Android)
+
+### 1) Treat empty native fetch as “no notification” (primary)
+
+**File:** `android/src/main/java/org/timesafari/dailynotification/FetchWorker.kt`
+
+**Issue:** `notificationContentsToDualPayloadBytes` must not turn an empty list into a non-empty payload if the product contract is “no rows in API → no notification.”
+
+**Direction:**
+
+- **Before:** Empty list → JSON `No updates` / `No new content` → success → chained notify scheduled.
+- **After (one of):**
+  - **A)** Return a dedicated sentinel payload (e.g. `{ "skipNotification": true }`) and teach **`NotifyReceiver` / worker** that resolves dual content to **not post** when that sentinel is present; **or**
+  - **B)** On empty list, **do not** call `DualScheduleNotifyScheduler.scheduleChainedNotifyAlarm` for this cycle (and optionally persist “last fetch had no content” for the helper); **or**
+  - **C)** Store an empty/marker cache row that `DualScheduleHelper.resolveDualContentBlocking` interprets as “return null” (no notification).
+
+Pick one strategy and keep behavior consistent with `relationship.fallbackBehavior`:
+
+- If `fallbackBehavior == "skip"`: skip notification when fetch returns empty or when sentinel indicates skip.
+- If `fallbackBehavior == "show_default"`: keep current default-title/body behavior **only** when the product intends it (may be wrong for TimeSafari).
+
+### 2) Honor `relationship.fallbackBehavior` end-to-end
+
+**Files:** `FetchWorker.kt`, `DualScheduleHelper.kt`, any worker/receiver that posts the dual notification.
+
+**Issue:** `DualScheduleHelper` reads `fallbackBehavior`, but the fetch path does not use the same semantics for “empty API result.”
+
+**Direction:** When persisting dual config, pass `fallbackBehavior` into the fetch success path so that **empty fetch + `skip`** never schedules or displays a notification.
+
+### 3) Tests
+
+- Dual fetch + native fetcher returns **empty list** → **no** notification posted (or no chained alarm), matching host expectation.
+- Non-empty list → notification with fetcher-provided title/body.
+- Optional: `fallbackBehavior` matrix (`skip` / `show_default`) with stale cache vs fresh cache.
+
+---
+
+## Host app follow-up (separate PR in `crowd-funder-for-time-pwa`)
+
+After the plugin implements empty-fetch semantics, set in `buildDualScheduleConfig` (`src/services/notifications/dualScheduleConfig.ts`):
+
+```ts
+relationship: {
+  autoLink: true,
+  contentTimeout: 5 * 60 * 1000,
+  fallbackBehavior: "skip", // was "show_default"
+},
+```
+
+Only do this once Android behavior matches the contract (otherwise users may get **no** notification even when you would want defaults on network failure — product decision).
+
+---
+
+## References in this repo (context only)
+
+- Host native fetcher returns no content when API `data` is empty: `android/app/src/main/java/app/timesafari/TimeSafariNativeFetcher.java` (`parseApiResponse`).
+- Host dual config today uses `fallbackBehavior: "show_default"`: `src/services/notifications/dualScheduleConfig.ts`.
+
+---
+
+## Plugin version verification
+
+- **Last verified against:** `@timesafari/daily-notification-plugin` **3.0.0** (`node_modules/.../package.json`).
+- **Prior builds:** Behavior matched the earlier **2.1.x** analysis; **2.2.0 → 3.0.0** did not remove the empty-list → synthetic JSON mapping or the chained-notify success path in the inspected sources.
+- Re-verify line numbers after rebasing or patching the plugin repo.