fix(swap): forward the quoted buy fee into the existing-currency swap transaction

[bmc08gt]

The bug

Every fee-bearing Get (buy) failed. DifferKt shows the server's transaction and the client's diverging on the VM instruction:

	opcode	accounts	data
server	0x14 VM::TransferForSwapWithFee	0, 17, 3, 8, 7, 5, 16, 12	14 + 500000 + 5000 + bump (18 bytes)
client	0x11 VM::TransferForSwap	0, 16, 3, 8, 7, 5, 12	11 + 500000 + bump (10 bytes)

The server's account list carries one extra entry — the fee destination at index 16 — so every client index at or above 16 is shifted down by one. That accounts for every other mismatched line in the diff; the signatures can never match.

Root cause

The ExistingCurrency branch of IntentStatefulSwap.transaction() never forwarded request.feeAmount to TransactionBuilder.swap, so it silently took that function's feeAmount = 0 default. The client quoted the fee to the server in the initiate request (LocalToProtobuf.setFeeAmount), then built the no-fee instruction anyway.

Everything downstream was already correct and tested — the proto parsing of feeDestination, the with-fee instruction builder, and the selection logic in buildExistingCurrencyBuyInstructions. Only the one call site was wrong. The NewCurrency and Stablecoin branches immediately below it both forward the fee, which is why only existing-currency buys broke.

The change

• Forward request.feeAmount?.underlyingTokenAmount?.quarks ?: 0 in the ExistingCurrency branch.
• Drop TransactionBuilder.swap's feeAmount = 0 default. It has exactly one non-test caller, so the same omission is now a compile error instead of a silently fee-less transaction.
• feeAmount is consumed only by the SwapRoute.Buy branch inside swap — buildSellInstructions and buildCrossCurrencyExistingSwapInstructions take no fee parameter — so this is a no-op for sells and cross-currency swaps.

Testing

ExistingCurrencyBuyFeeTest is a JVM unit test, so it runs in CI — the existing swap coverage lives in androidTest, which does not. The key case drives the real IntentStatefulSwap with a real StatefulSwapRequest at the amounts from the failing log (500000 swap / 5000 fee) and asserts the transaction carries opcode 0x14 with both amounts encoded.