docs: testing.md with testcases to keep in mind

docs/testing.md

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+# Testing Scenarios (Validation Apps)
+
+These apps test HLC ordering, gossip convergence, and conflict resolution.
+
+## Level 1: Pixel Board (Visual Convergence)
+
+50x50 collaborative grid where users paint pixels.
+
+**Data Model:** `/canvas/{x}_{y}` → `{hex_color}`
+
+**Tests:**
+- Visualize sync disagreements immediately
+- High write volume (log performance)
+- Simultaneous writes (HLC tiebreaker)
+
+**Scenario:** Node A paints all red (offline), Node B paints all blue (offline), connect. Board must be identical on both.
+
+---
+
+## Level 2: Shared Grocery List (LWW Trap)
+
+List with add/check/delete operations.
+
+**Data Model:** `/list/{item_uuid}` → `{ name, status: "needed"|"bought" }`
+
+**Tests:** Exposes LWW weakness (resurrection bug)
+
+**Scenario:**
+1. Alice syncs, sees "Milk", goes offline, marks "bought"
+2. Bob syncs, sees "Milk", deletes it
+3. Reconnect
+
+**Result:** Item either resurrects or vanishes based on timestamp. Forces tombstone pattern.
+
+---
+
+## Level 3: Chat Room (Causal Ordering)
+
+Group chat application.
+
+**Data Model:** `/chat/{channel}/{timestamp}_{node_id}` → `{ msg }`
+
+**Tests:**
+- HLC causal ordering
+- Prefix queries (redb range scans)
+- Gap detection via vector clocks
+
+**Scenario:**
+1. Node A sends "Msg 1"
+2. Node B sees it, replies "Msg 2"
+3. Node C comes online, connects only to B
+
+**Success:** Node C receives "Msg 1" before/with "Msg 2" (transitive sync).
+
+---
+
+## Level 4: Chaos Monkey (Automated Simulation)
+
+Tokio-based simulation harness with in-memory networking.
+
+**Setup:**
+- 5 node threads in one process
+- In-memory network (tokio channels)
+- Chaos monkey randomly: cuts connections, writes random keys, sleeps threads
+
+**Assertion:**
+```rust
+let state_0 = nodes[0].dump_state_hash();
+for i in 1..5 {
+    assert_eq!(state_0, nodes[i].dump_state_hash());
+}
+```
+
+Catches HLC clamping edge cases that manual testing misses.