fix(sync): handle piggyback limit in high-volume sync

When syncing 100+ operations, the server's piggyback limit (100 ops)
was causing Client B to miss operations. The server returned 100 piggybacked
ops but latestSeq was set to the actual server sequence (e.g., 199).
Client B then updated lastServerSeq to 199, so subsequent download got 0 ops.

Changes:
- Server: Add hasMorePiggyback flag to UploadOpsResponse when piggyback limit
  is reached and more ops exist
- Client: When hasMorePiggyback is true, store lastServerSeq as the max
  piggybacked op's serverSeq instead of latestSeq, ensuring subsequent
  download fetches remaining ops
- Effects: Change switchMap to mergeMap in autoAddTodayTagOnMarkAsDone to
  ensure ALL mark-as-done actions trigger planTasksForToday
- Flush service: Implement two-phase wait strategy (poll queue + acquire lock)
  to ensure all pending writes complete before upload
- Add diagnostic logging for operation counts at key stages

Test: High volume sync with 50 tasks + 49 mark-as-done (197 ops total)
now correctly syncs all done states via piggyback (100) + download (97).

docs/ai/high-volume-sync-investigation.md

@@ -0,0 +1,80 @@
+# High Volume Sync Investigation
+
+## Summary
+
+Investigation into sync failures when synchronizing 100+ operations between clients.
+
+**STATUS: RESOLVED** - Root cause identified and fixed on 2025-12-22.
+
+## Problem Description
+
+When Client A creates many tasks (50+) and marks most as done, then syncs to Client B:
+
+- All tasks appear on Client B
+- But only a subset of "done" states are applied (16 out of 49 expected)
+
+## Root Cause
+
+**Server Piggyback Limit**: The server's upload endpoint limits piggybacked operations to 100, but returns `latestSeq` as the actual server sequence (e.g., 199). The client then updates its `lastServerSeq` to 199, thinking it has all ops. When it subsequently downloads, it gets 0 ops because there are none after seq 199.
+
+## Fix Applied
+
+### 1. Server-side: Added `hasMorePiggyback` flag
+
+**File**: `packages/super-sync-server/src/sync/sync.routes.ts`
+
+When the piggyback limit (100) is reached and more ops exist on the server:
+
+- Server returns `hasMorePiggyback: true` in the response
+- Logs: `Piggybacking N ops (has more: true, lastPiggybackSeq=X, latestSeq=Y)`
+
+### 2. Client-side: Handle `hasMorePiggyback`
+
+**Files**:
+
+- `src/app/pfapi/api/sync/sync-provider.interface.ts` - Added `hasMorePiggyback` to `OpUploadResponse`
+- `src/app/core/persistence/operation-log/sync/operation-log-upload.service.ts`:
+  - When `hasMorePiggyback` is true, set `lastServerSeq` to max piggybacked op's serverSeq instead of `latestSeq`
+  - The subsequent download will then fetch the remaining ops
+
+### Earlier Fix: `switchMap` → `mergeMap` (still needed)
+
+**File**: `src/app/features/tasks/store/task-related-model.effects.ts`
+
+Changed `switchMap` to `mergeMap` in `autoAddTodayTagOnMarkAsDone` effect to ensure ALL mark-as-done actions trigger `planTasksForToday`, not just the last one.
+
+## Test Results
+
+After fix:
+
+```
+[Client B] Received 100 piggybacked ops (more available on server)
+[Client B] hasMorePiggyback=true, setting lastServerSeq to 100 instead of 199
+[Client B] Downloaded ops breakdown: {UPD: 99}
+[Client B] Applied and marked 99 remote ops
+[HighVolume] DOM shows 49 done tasks
+1 passed
+```
+
+All 49 done states now sync correctly!
+
+## Files Modified
+
+1. `packages/super-sync-server/src/sync/sync.types.ts` - Added `hasMorePiggyback` to `UploadOpsResponse`
+2. `packages/super-sync-server/src/sync/sync.routes.ts` - Detect and return `hasMorePiggyback` flag
+3. `src/app/pfapi/api/sync/sync-provider.interface.ts` - Added `hasMorePiggyback` to client-side type
+4. `src/app/core/persistence/operation-log/sync/operation-log-upload.service.ts` - Handle `hasMorePiggyback`
+5. `src/app/features/tasks/store/task-related-model.effects.ts` - switchMap → mergeMap fix
+
+## Running the Tests
+
+```bash
+# Run stress tests
+npm run e2e:supersync:file tests/sync/supersync-stress.spec.ts
+
+# Run with verbose logging
+E2E_VERBOSE=1 npm run e2e:supersync:file tests/sync/supersync-stress.spec.ts
+
+# Run only high volume test
+npm run e2e:supersync:file tests/sync/supersync-stress.spec.ts --grep "High volume"
+```