[executor] cross-chain chain_id partitioning is dead code in v0.1 BSC-only scope

[obchain]

PR: #45 (feat/20-multi-liq-batcher)
File: crates/charon-executor/src/batcher.rs, plan() and test different_chains_produce_separate_batches

plan() partitions opportunities by chain_id using a HashMap. CLAUDE.md states: 'Venus protocol on BNB Chain only. Multi-chain/multi-protocol expansion is a config change later, not a rewrite.' In v0.1, all scanner-produced opportunities will have chain_id = 56. The per-chain partitioning logic and the test exercising chain_id = 1 (Ethereum mainnet) are dead code for the current scope.

Impact: Dead code increases maintenance surface. The test different_chains_produce_separate_batches passes chain_id = 1 opportunities that will never appear in v0.1 scanner output, creating false coverage confidence for an untested and out-of-scope code path. Any bug in cross-chain ordering or grouping will be silently carried until multi-chain expansion.

Fix: Remove the HashMap partitioning and the cross-chain test. Replace with a direct assertion that all input opportunities share chain_id = 56 (returning an error or filtering for anything else), deferring the multi-chain expansion to the milestone that introduces it. This matches CLAUDE.md's 'config change later, not a rewrite' model.

Refs #45

[obchain]

Resolved on feat/20-multi-liq-batcher in 7080206. HashMap<chain_id, _> partitioning removed. Batcher::plan now rejects any opportunity whose chain_id != 56 via BatcherError::UnsupportedChain; v0.1 scope is BSC-only. Tests: plan_rejects_non_bsc_chain_id, assert_single_chain. different_chains_produce_separate_batches deleted.