feat: implement real DEX router integration for swap_and_pay

[Pvsaint]

Problem

dex_router.rs contains only a stub implementation. Both swap_exact_tokens_for_tokens and swap_tokens_for_exact_tokens are no-ops: they do not transfer tokens, do not enforce slippage, and emit event symbols without actually publishing them. Additionally, SwapAndPayArgs is defined in lib.rs but there is no swap_and_pay entry point on PaymentProcessor.

// Current stub — does nothing
pub fn swap_exact_tokens_for_tokens(...) -> Vec<i128> {
    soroban_sdk::Symbol::new(&env, "SWAP"); // no-op
    ...
}

Required Work

1. Implement swap_and_pay on PaymentProcessor using SwapAndPayArgs
2. Call the external DEX router contract via cross-contract invocation using DexRouterClient
3. Enforce amount_out_min slippage guard — panic/error if output < minimum
4. Enforce deadline — reject if env.ledger().timestamp() > deadline
5. After swap succeeds, create the payment via normal create_payment flow
6. Emit SWAP/EXECUTED event with correct data payload

Acceptance Criteria

• swap_and_pay is callable on PaymentProcessor and accepts SwapAndPayArgs
• Token transfer from payer to router is executed via the token contract
• amount_out_min slippage is enforced; returns InvalidAmount error if violated
• deadline is enforced; returns PaymentExpired error if past
• A SWAP/EXECUTED event is emitted with (stream_id, sender, amount_in, amount_out) data
• Integration test covers: happy path, slippage exceeded, expired deadline
• EVENTS.md is updated with the SWAP/EXECUTED event definition

Branch & Commit Examples

branch: feat/swap-and-pay
commit: feat(payment-processor): implement swap_and_pay with DEX router integration