docs: add SDS integration test plan

SDS_TEST_PLAN.md

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+# Minimal SDS Integration Test Plan
+
+## Overview
+This branch implements a minimal proof-of-concept for SDS (Scalable Data Sync) integration in OpChan, focusing exclusively on vote consistency.
+
+## What Changed
+
+### 1. Core SDS Implementation
+- **New Package**: Added `@waku/sds` dependency
+- **MinimalSDSWrapper**: Basic implementation at `src/lib/waku/sds/minimal-sds.ts`
+  - Manages Lamport timestamps for causal ordering
+  - Tracks causal history (last 3 message IDs)
+  - Provides `isCausallyNewer()` comparison
+
+### 2. Vote Message Enhancement
+- Vote messages now include SDS metadata:
+  ```typescript
+  {
+    sds: {
+      channelId: "opchan:votes:all",
+      lamportTimestamp: number,
+      causalHistory: string[]
+    }
+  }
+  ```
+
+### 3. Conflict Resolution
+- When receiving votes, the system now:
+  1. Checks if a vote already exists for the same target+author
+  2. Uses Lamport timestamps to determine which is newer
+  3. Only updates if the incoming vote is causally newer
+  4. Logs all decisions for debugging
+
+### 4. Visual Indicators
+- Added SDS status badge in header showing "SDS: Active (Votes)"
+- Debug console logs show SDS operations
+
+## Test Scenarios
+
+### Scenario 1: Basic Vote Ordering
+1. User A upvotes a post
+2. User A changes to downvote
+3. Verify the downvote is applied (newer Lamport timestamp)
+
+### Scenario 2: Concurrent Votes
+1. Open two browser tabs with same wallet
+2. Vote differently in each tab quickly
+3. Verify the last vote wins based on causal ordering
+
+### Scenario 3: Network Delays
+1. Vote on a post
+2. Disconnect network
+3. Change vote
+4. Reconnect
+5. Verify votes are ordered correctly when synced
+
+### Scenario 4: Vote History
+1. Vote multiple times on same content
+2. Check console logs for SDS debug messages
+3. Verify causal history is maintained
+
+## Debug Output
+
+Enable browser console to see SDS operations:
+```
+[SDS HH:MM:SS] VOTE SENDING - postId:userAddress
+[SDS HH:MM:SS] VOTE NEW - postId:userAddress
+[SDS HH:MM:SS] VOTE UPDATED - postId:userAddress (old: 5, new: 7)
+[SDS HH:MM:SS] VOTE IGNORED_OLDER - postId:userAddress
+```
+
+## Verification
+
+### Console Commands
+```javascript
+// Check current vote state
+messageManager.messageCache.votes
+
+// Check if message has SDS metadata
+messageManager.messageCache.votes['postId:address'].sds
+```
+
+### Expected Behavior
+- Votes should never "flip back" to older states
+- Lamport timestamps should increase monotonically
+- Causal history should contain previous vote IDs
+
+## Limitations
+
+This minimal implementation:
+- Only applies to votes (not posts, comments, or moderation)
+- Uses a single channel for all votes (not optimal for scale)
+- Doesn't implement full SDS features (sync messages, bloom filters)
+- No persistence of SDS state between sessions
+
+## Next Steps
+
+If this test is successful:
+1. Extend to comments for thread ordering
+2. Add per-cell channels for better scalability
+3. Implement sync messages for reliability
+4. Add persistence of channel state
+5. Implement gap detection and recovery