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