examples: expand examples/ with common dApp patterns

[satyakwok]

README points users at the SDK + WebSocket subscription helpers, but the surface is more useful with copy-paste-ready examples for the most common operations.

Why

First-touch developers come for "how do I connect to Sentrix and do X". The SDK API surface is large; without runnable examples, they have to read the source.

Scope

• examples/connect-and-read-block.ts — minimal connect + getBlock
• examples/send-eth-tx.ts — sign + send + wait for receipt
• examples/erc20-transfer.ts — ERC-20 transfer via canonical contracts (WSRX)
• examples/subscribe-newheads.ts — WebSocket newHeads subscription
• examples/multicall.ts — batched reads via Multicall3 from canonical-contracts
• Each example self-contained, runnable with pnpm tsx examples/<file>.ts

Acceptance

• 5 examples committed under examples/
• README links each from a new "Examples" section