Add shared_infrastructure example and update docs

- Add new `shared_infrastructure` example demonstrating multi-wallet
  resource sharing pattern
- Update `trading_client` example to show both initialization methods
- Update README with:
  - New feature: "Shared Infrastructure" in features list
  - Both methods (simple and shared) in TradingClient creation section
  - New example in examples summary table
- Add `shared_infrastructure` to workspace members

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/shared_infrastructure/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "shared_infrastructure"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+sol-trade-sdk = { path = "../.." }
+solana-sdk = "3.0.0"
+solana-commitment-config = { version = "3.0.0", features = ["serde"] }
+tokio = { version = "1", features = ["full"] }