[evm]: cross-chain partial fills for IntentGatewayV2

[seunlanlege]

Summary

Adds partial fills for cross-chain orders to IntentGatewayV2. Previously cross-chain fills were all-or-nothing; this lets multiple solvers each fill a slice of a cross-chain order, mirroring the existing same-chain partial-fill behaviour.

Design

The core invariant: every escrow movement uses a single monotonic _cumulativeReleased(escrowTotal, filled, totalRequired) function, so across any split — and regardless of cross-chain message arrival order — the redeemed slices plus any cancel refund sum to exactly the escrowed amount, with rounding dust deterministically landing in the completing fill.

• _fillCrossChain — partial-aware: accumulates _partialFills, pays the beneficiary pro-rata, clears _filled on partial / keeps it on completion, and dispatches RedeemEscrowPartial (non-finalizing) or RedeemEscrow (finalizing). Output-calldata orders still require single-fill completion (PartialFillNotAllowed).
• onAccept — new RedeemEscrowPartial kind releases a proportional slice without finalizing (escrow stays open, fee pot left for the completing redeem). The completing RedeemEscrow finalizes and forwards the fee pot (completing-solver-takes-all).
• _cancelFromSource / onGetResponse — proves each output's _partialFills on the destination via a GET request and refunds only the proven-unredeemed fraction, never the raw remaining escrow, so in-flight redeems stay covered. The deadline gate makes the post-deadline proof a final snapshot.
• _cancelFromDest — reads _partialFills locally and refunds the unredeemed fraction (no proof needed; freeze + snapshot are atomic).

Escrow proofs match values to inputs by storage key (GET responses come back sorted by key, not request order). EscrowReleased now fires for every source-side release (including partial redeems) and carries the solver. placeOrder now enforces inputs.length == outputs.length and rejects zero-amount outputs (both would otherwise strand escrow).

Testing

forge test --match-contract IntentGatewayV2 and the reentrancy suite — 125 tests pass. New tests cover: proportional release + two-solver completion, non-finalizing source redeem, cancel refunding the unredeemed fraction, in-flight-redeem-after-cancel consistency, the fully-filled-cancel race (refund 0, fee pot withheld for the completing solver), multi-token value-by-key matching, double-cancel idempotency, the calldata guard, the placeOrder validations, and a _partialFills storage-slot guard.

Follow-ups (out of scope)

• TS SDK: handle RedeemEscrowPartial + cross-chain PartialFill, construct partial cross-chain fills.
• Substrate intents-coprocessor partial support.
• Independent/cloud review recommended before mainnet (cross-chain fund movement).

[seunlanlege]

Would be cool to add simplex, sdk & indexer support to this PR as well