Phase 1: Rust-native Platform Actor for child subnets

[LePremierHomme]

Replace fendermint_vm_interpreter::fvm::state::ipc::GatewayCaller with a new system actor in

fendermint_vm_actor_interface called Platform Actor with minimal, consensus-related functionality for the subnet network as a child subnet.

This means that the IPC subnet should be instantiated without using the following Solidity artifacts dependency:

• GatewayGetterFacet
• CheckpointingFacet
• TopDownFinalityFacet
• XnetMessagingFacet
• GatewayManagerFacet

Main Functionality

Module	Feature	Current Module	Current Function
Checkpointting	commit bottom-up checkpoint	CheckpointingFacet	.commitCheckpoint()
Topdown	commit to parent finality	TopDownFinalityFacet	.commitParentFinality()
Topdown	store validator change requests	TopDownFinalityFacet	.storeValidatorChanges()
Topdown	apply topdown committed changes	TopDownFinalityFacet	.applyFinalityChanges()
GMP	system actor applies incoming cross-net messages	XnetMessagingFacet	.applyCrossMessages()

Testing:

• Port relevant Solidity integration tests to Rust
• E2E tests to check against regression for the limited child-subnet functionality (checkpoint submission, topdown finality, inbound cross-net messages)

[linear]

IPC-377