[louiss72]

Summary

• add StateChangeHandler / WebSocketRelayerOptions.onStateChange
• add relayer.onStateChange(handler) for event-driven connection state transition subscriptions
• emit transitions for connecting, connected, disconnected, reconnecting, and destroyed only when the transition changes
• cover the public subscription API with a relayer regression test

Closes #218.

Validation

• npm test -- src/tests/relayer-stress.test.ts -> 1 file / 13 tests passed
• npm run build -> passed
• npm run lint -> passed
• git diff --check -> passed
• npm run typecheck is blocked by an existing WalletConnect edge-case test issue unrelated to this change: chainId: undefined is not assignable with exactOptionalPropertyTypes
• npm test is blocked by an existing pagination cursor expectation in src/tests/pagination-cursor.test.ts; the focused relayer suite passes

Payout

This is Stellar Wave / USDC-compatible. EVM/Base USDC address: 0x2f8081562ac67467d1cbd40ab3120849c1f587da.

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