Support digest-backed state expansion for open agents

[michaelsutton]

This PR adds digest-backed virtual slots for open-agent apps, and turns the old Open Lattice sketches into a compiling Open ICC lattice fixture.

Changes:

• adds virtual <slot>; on base states
• adds state X expands Base { slot: Memory; } bindings for concrete agent states
• keeps the stored covenant layout identical to the base state
• exposes expanded memory through slot-qualified Argent source, e.g. strategy.hunger
• requires typed slot payload constructors, e.g. strategy: ForagerStrategy { ... }
• lowers expanded memory reads through hidden fixed-size preimages
• repacks mutated slot payloads back into the backing digest
• teaches argent-runtime to accept slot-qualified source values and fill hidden preimage witnesses
• allows expanded actors, like ForagerState expands AgentCapsule, to satisfy observed actor<AgentCapsule> handles
• moves AgentCapsule into core.ag; agent.ag imports the core state view
• replaces the old examples/open_lattice sketches with the real examples/open_icc fixture
• regenerates tracked Open ICC build outputs
• adds an execution test for Forager::step, including controller co-spend authorization and digest-backed strategy repacking

Example:

state AgentCapsule {
    covid controller_id;
    virtual strategy;
    int energy;
}

state ForagerStrategy {
    int hunger;
    int mood;
}

state ForagerState expands AgentCapsule {
    strategy: ForagerStrategy;
}

actor Forager owns ForagerState {
    entry step() emits {
        agent: Forager;
    } {
        ForagerState next_agent = {
            controller_id: controller_id,
            strategy: ForagerStrategy {
                hunger: strategy.hunger + 1,
                mood: strategy.mood,
            },
            energy: energy - 1,
        };

        become agent <- Forager(next_agent);
    }
}

The generated Sil stores the base layout only: controller_id, strategy, and energy. The ForagerStrategy preimage is Argent runtime/compiler plumbing: the contract verifies it against the stored strategy digest, exposes strategy.hunger and strategy.mood to source code, and writes mutations back as a new digest.