feat(cardinal): add struct system registration

[smsunarto]

Summary

• add cardinal.System and cardinal.RegisterSystemV2 for caller-owned system instances
• require a non-nil pointer to a struct with BaseSystemState and a pointer-receiver Run() method
• initialize exported Cardinal dependencies while preserving constructor-owned private dependencies and scratch memory
• keep the existing RegisterSystem API and behavior unchanged
• fail early for private or pointer-form Cardinal dependency fields

Developer experience

type MovementSystem struct {
    cardinal.BaseSystemState
    Positions PositionSearch

    runtime *Runtime
    scratch []Input
}

func (s *MovementSystem) Run() {
    // Use initialized Cardinal fields and persistent private state.
}

cardinal.RegisterSystemV2(world, NewMovementSystem(runtime))

The constructor owns external dependencies. Cardinal initializes the same instance and calls Run() on it for every configured hook.

Validation

• go test ./pkg/cardinal -count=1
• scoped golangci-lint: 0 issues
• pointer/value validation, persistent state, private dependency, and hook tests

Stack

This is the base PR for NativeAOT runtime PR #919.