feat(bridge): Ethereum mainnet sender deployment script

[ByteYue]

Summary

• Adds scripts/eth-mainnet-sender/DeployBridge.s.sol — single-broadcast forge script that deploys GravityPortal + GBridgeSender to Ethereum mainnet and hands ownership of both to a Safe multisig via Ownable2Step.transferOwnership() in the same broadcast.
• Script hard-codes the verified mainnet G token (0x9C7BEBa8F6eF6643aBd725e45a4E8387eF260649), enforces chainid == 1 (with an ALLOW_NON_MAINNET=1 escape for fork tests), and runs 7 post-deploy invariant asserts.
• Adds a minimal script-side README.md that points to the full operator runbook in mono-grav/docs/mainnet/ETH-BRIDGE-SENDER-DEPLOYMENT.md.

Design notes

• Deployer EOA is temporary owner for the duration of the script only; transferOwnership runs in the same broadcast immediately after each deploy, so the EOA window is effectively zero.
• feeRecipient is set directly to the multisig in the constructor, so even if the EOA window were attacked, any withdrawFees() still routes ETH to the multisig.
• GBridgeSender receives the GravityPortal address from the same script context, eliminating manual transcription risk.

Test plan

• forge build clean on Solc 0.8.30 with via_ir
• Fork dry-run on https://ethereum-rpc.publicnode.com — 4 txs simulated, 7 invariants pass, deployment JSON produced
• Gas estimate ~1.73M; at 30 gwei ≈ 0.05 ETH deployer budget
• Real mainnet deploy (requires multisig address + final fee values — tracked in the runbook)

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