[indexer]: index the gateway's OrderCancelled event

[Wizdave97]

09888bd1 added OrderCancelled(bytes32 indexed commitment, address canceller) to IntentGatewayV2. It was the only contract event with no counterpart here — the ABI already carried the other reshaped events from this cycle (OrderFilled/PartialFill/ EscrowReleased/EscrowRefunded with their token arrays, DeploymentAdded, DestinationProtocolFeeUpdated), so cancellation was the whole gap.

Adds a CANCELLED status, an IOrderV3Cancellation entity, a handler, and the datasource wiring. canceller is stored separately from the order's user because the destination-side cancel route is permissionless once the order has expired, so the two are not the same account in general.

recordOrderCancellation advances the status only from PLACED. OrderCancelled marks the initiation of a cancellation, not its completion — EscrowRefunded stays terminal and still owns REFUNDED. Since updateOrderStatus assigns without comparing against the current value, and a cross-chain cancel is initiated on the destination chain while its refund lands on the source chain via a separate datasource with no ordering guarantee between them, an unguarded write would let a late-indexed cancellation move a settled order back to CANCELLED. Guarding on PLACED makes it idempotent and order-independent.

The guard is local to the new method rather than a general never-regress rule in updateOrderStatus: the broader change would alter every existing transition on a path with no ordering test coverage, and deserves its own justification.

An order resting at CANCELLED is an expected steady state, not an indexing gap — the source-side route re-emits on every call and only refunds when the GET response returns.