feat(pools): Balancer V2 pool pricing via queryBatchSwap (Phase 1, draft)

[jenish-25]

Summary

Adds a BalancerV2Pool that implements the Pool trait and prices swaps by simulating the Balancer Vault's queryBatchSwap inside revm — modeled on the existing CurvePool (EVM-simulation) pattern rather than porting Balancer's weighted/stable math to Rust.

PoolProtocol::BalancerV2 / PoolClass::BalancerV2 already existed and the Vault ABI was already vendored (used for flash loans); this wires them up for pricing.

Scope: Phase 1 (pricing & detection) only. Execution (multicaller opcode encoder + on-chain Vault routing) is intentionally deferred — get_abi_encoder returns None for now.

What's included

• crates/defi/pools/src/balancerv2pool.rs — BalancerV2Pool + Pool impl
  • calculate_out_amount / calculate_in_amount build a single-hop queryBatchSwap (GIVEN_IN / GIVEN_OUT) and run it against the state DB via evm_call, decoding the int256[] deltas
  • get_state_required adds a representative queryBatchSwap per direction so the prestate tracer captures Vault + pool storage (singleton-Vault layout)
• crates/defi/abi/src/balancer/pool.rs — minimal IBalancerV2Pool interface (getPoolId, getSwapFeePercentage); #[sol(rpc)] added to the Vault interface for fetch_pool_data
• crates/defi/address-book — Balancer V2 Vault address constant

Status — draft, not yet ready to merge

Opening as a draft for early feedback on the approach. Two things outstanding:

1. Pricing path not yet verified against a live chain. The fork integration test (test_balancer_v2_pool, #[ignore]) is compile-checked but hasn't been run against an archive node yet. The open question is whether queryBatchSwap returns int256[] cleanly under evm_call or reverts-with-data in query mode — the decode may need adjusting once run.
2. Pool discovery is not wired into the loader graph — a BalancerV2Pool is constructible and priceable, but I deferred the discovery mechanism (dedicated loader vs. config-based) pending a preference.

Happy to adjust the approach, split it up, or hold it — feedback welcome.

Test Plan

• cargo build on affected crates
• Unit tests: queryBatchSwap calldata roundtrip + delta-sign decoding
• clippy clean on the new file; formatting clean
• Run test_balancer_v2_pool against a mainnet archive node (MAINNET_WS) to confirm the pricing path end-to-end
• Wire pool discovery once the mechanism is agreed

[jenish-25]

Design discussion and open questions: #11

[jenish-25]

Heads up on the failing checks — they're pre-existing on main, not introduced by this change:

• cargo deny check: fails to parse a malformed upstream advisory (RUSTSEC-2026-0124.md, TOML parse error in the RustSec DB) — unrelated to this repo's dependencies. This PR changes zero dependencies.
• make clippy: nightly clippy::unnecessary_sort_by now errors on pre-existing kabu-strategy-merger code under -D warnings. Fixed separately in fix(merger): use sort_by_key to satisfy clippy::unnecessary_sort_by #12.
• make test / make swap-test-all: need node access (MAINNET_WS) that isn't available to fork PRs via pull_request_target.

For reference, the README-only PR #88 fails the same four jobs, and main's CI is currently red for the same reasons.