Phase 4: Outbound GMP in Platform Actor

[LePremierHomme]

Extend Platform Actor to support outbound GMP from child subnets, enabling child-to-parent and sibling-to-sibling messaging.

Module	Feature	Current Module	Current Function
GMP	send general-purpose cross-net msg (from contracts only)	GatewayMessengerFacet	.sendContractXnetMessage()
GMP	commit validated cross-net msgs	LibGateway	.commitValidatedCrossMessage()
GMP	check msgs	LibGateway	.checkCrossMessage()

Testing

• Integration / e2e tests for child-to-parent
• Integration / e2e tests for sibling-to-sibling

[linear]

IPC-380