feat: harden trading SDK examples

examples/nonce_cache/README.md

@@ -0,0 +1,17 @@
+# Durable Nonce
+
+[中文](README_CN.md)
+
+Shows how to fetch a durable nonce and attach it to a PumpFun transaction triggered by a `sol-parser-sdk` gRPC event.
+
+> This is a live-transaction template. Set `PRIVATE_KEY`, replace `use_your_nonce_account_here`, and configure RPC and gRPC before running. The nonce authority must match the signer.
+
+```bash
+export GRPC_ENDPOINT=https://your-yellowstone.example
+export GRPC_AUTH_TOKEN=optional-token
+cargo run --package nonce_cache
+```
+
+Do not set both a recent blockhash and a durable nonce in new code; use `SimpleBuyParams::with_durable_nonce`. Refresh nonce state immediately before construction, and never reuse consumed nonce state. A nonce extends transaction validity but does not preserve quote freshness.
+
+See [Nonce Cache](../../docs/NONCE_CACHE.md).