[executor] Transaction builder: EIP-1559 gas + eth_call simulation gate + nonce management

[obchain]

Context

The last stop before submission. Encodes the call, prices the gas, dry-runs it via eth_call, and hands off to the signer. BSC uses EIP-1559 (since the Paris hard fork). The simulation gate catches ~99% of races pre-submit so we don't burn gas on losing bids.

Scope

• New crates/charon-executor/ crate
• TxBuilder::build(opportunity) -> SignedTransaction
• Encodes CharonLiquidator.executeLiquidation(...) calldata from LiquidationOpportunity
• EIP-1559 gas pricing: max_fee_per_gas = base_fee × 1.2, max_priority_fee from config
• eth_call simulation: if the call reverts, abort with the revert reason (no tx sent)
• NonceManager: monotonic nonce tracking so concurrent liquidations don't collide
• Skip-submission flag for now (signing comes with wallet setup)

Acceptance criteria

• charon-executor crate compiles
• TxBuilder::build returns a fully-encoded, simulatable tx
• Simulation reverts produce structured error logs with the revert reason
• NonceManager returns monotonically-increasing nonces concurrently
• EIP-1559 fields (maxFeePerGas, maxPriorityFeePerGas) populated correctly

References

• PRD section 5c (Tx Builder)
• EIP-1559: https://eips.ethereum.org/EIPS/eip-1559