[codex] Move swap router empty-list guards before folds

[caydyan]

Summary

• Move empty swap-list assertions before the fold in swap-multi, swap-x-for-y-same-multi, and swap-y-for-x-same-multi.
• Tighten router tests so empty lists assert ERR_EMPTY_SWAPS_LIST directly.
• Add empty-list coverage for both same-token multi-swap entrypoints.

Audit context

This addresses Sonic Mast's Bitflow dlmm-swap-router-v-1-1 audit R01 from https://gist.github.com/sonic-mast/c3a352aa6b9b46dd42e36f163d9d9b2d. The finding calls out that empty-list checks execute after the fold. Current main no longer has the audited swap-simple-multi entrypoint, so this PR fixes the still-present affected entrypoints.

Why

The prior behavior was functionally safe for empty lists because folding an empty list returns immediately, but the check read as a post-fold condition. Moving it before the fold makes the empty-list guard an explicit precondition and prevents future changes to fold bodies from accidentally placing any work before the guard.

Validation

• git diff --check
• clarinet check contracts/dlmm-swap-router-v-1-1.clar

Additional validation attempted:

• clarinet check for the full project currently fails outside this change on the existing mock-pool / dlmm-pool-trait get-pool-for-add signature mismatch.
• npx vitest run tests/routers/swap-router.test.ts currently fails before collecting tests because the checked-in deployment plan uses a transaction-type format that this local Clarinet/Vitest stack cannot read.

BTC payout address if this is accepted for any related bounty: bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc