[executor] Multi-liquidation batcher: group same-chain opportunities into a single tx

[obchain]

Context

When multiple Venus positions become liquidatable in the same block (common after a large oracle update), submitting them as one batched tx amortizes the base cost and wins the race against competitors submitting one-off txs.

Scope

• Extend CharonLiquidator.sol with a batchExecuteLiquidations(LiquidationParams[] calldata) entry point, or
• Add a sibling CharonMulticall.sol contract for batching
• Rust side: Batcher in charon-executor/ that groups opportunities per block per chain
• Gas cap per batch (configurable) so we don't overshoot block gas limit
• Fall back to individual txs if any batch member fails simulation

Acceptance criteria

• Solidity contract compiles and passes fork tests for 2- and 3-position batches
• Rust Batcher groups by chain correctly when multiple opportunities arrive in one block
• Batch gas estimation stays under the configured cap
• Failed simulation on one member logs WARN and proceeds with the rest

References

• PRD section 6c (Multi-Liquidation Batching)