Allow 7702 cross-contract calls to target a specific ephemeral account

[mattgle]

Funds can be stranded on an ephemeral account once the index rotates past it — a swap that reverts on slippage leaves them behind. The 7702 cross-contract entry points resolve the ephemeral account internally through getCurrentEphemeralAddress, so a caller cannot build a batch against the account that actually holds the funds and they stay unreachable.

This adds an optional trailing ephemeralIndex to gasEstimateForUnprovenCrossContractCalls7702 and generateCrossContractCallsProof7702. When set, the unshield recipient, the EIP-7702 authorization authority and the nonce all resolve to that one account, reusing the engine's existing ephemeralWalletOverride scoped in a try/finally so the shared override is always restored. Omit it and behaviour is byte-identical to today.

Draft for your call on two things: whether you want this in #131 rather than stacked on it, and whether the unshield paths (tx-unshield, tx-unshield-base-token-7702, tx-proof-unshield) should take the same parameter — they still hard-wire the current account.

[tom332]

[BUG] Critical Fallback Routing Failure: 50,551 USDC Trapped in Relay Adapt Contract via Safe Unshield Loop
#140

[tom332]

Please help me; there is a critical bug in Railway Wallet that caused me to lose 50,000 USDC. I reported this to their development team 20 days ago, but they have completely ignored the issue—neither fixing the code nor compensating me. One of their nodes transferred my 50,000 USDC into their own wallet and refuses to return it. Please check #140.