examples: add examples/ folder with copy-paste recipes

[satyakwok]

README describes the 4 client doors (native REST / EVM via alloy / gRPC via tonic / wallet+signing via secp256k1). Add runnable examples so developers can clone the repo + cargo run --example <name> to see each door in action.

Scope (one example per door, plus 2 multi-door)

• examples/native_get_block.rs — REST client basic read
• examples/evm_send_tx.rs — alloy provider transfer
• examples/grpc_stream_blocks.rs — tonic streamBlocks subscription
• examples/wallet_sign_and_send.rs — wallet + EVM send
• examples/dex_swap.rs — multi-door: read pair reserves, sign + send swap tx
• examples/multicall_batched_reads.rs — Multicall3 from canonical-contracts

Acceptance

• 6 examples committed
• cargo run --example <name> works on a fresh clone
• README "Examples" section linking each