docs: clarify websocket snapshot ordering

README.md

@@ -78,6 +78,8 @@ Run the WS hot-path benchmark locally with `cargo bench --bench ws_hot_path`.
 
 **Parsing paths:** `polyfill-rs` keeps two parsing layers on purpose. The allocation-sensitive WS `book` path uses `WsBookUpdateProcessor` in `src/ws_hot_path.rs`, which walks a reusable `simd-json` tape and applies fixed-point book levels directly. The generic stream parser in `src/decode.rs` is an ergonomic compatibility path: it parses through `serde_json::Value` so it can tolerate batches, unknown event types, and mixed message shapes. Likewise, several generic numeric/decimal deserializers in `src/decode.rs` accept string-or-number API fields through `serde_json::Value`; they are not the zero-allocation hot path.
 
+**WebSocket snapshot ordering:** Polymarket `book` messages expose a millisecond `timestamp` and optional `hash`, but no monotonic server sequence/version. The book applier treats newer timestamps as newer, rejects older timestamps, suppresses exact same-timestamp/same-hash duplicates, and accepts same-timestamp/different-hash snapshots in websocket arrival order. The hash distinguishes duplicate vs distinct state; it is not a logical ordering key.
+
 **Key Performance Optimizations:**
 
 The 21.4% performance improvement comes from HTTP/2 tuning with 512KB stream windows optimized for 469KB payloads, explicit Polymarket request headers, SIMD-backed parsing where the client uses the typed fast-response helper for large REST market responses, and opt-in connection prewarming/keep-alive support.