test(fc): store initialization from non-genesis anchor (checkpoint sync)

[tcoratger]

Context

Clients don't always start from genesis. Checkpoint sync lets a node start from a recent finalized state and block. The fork choice store is initialized via `Store.from_anchor(anchor_state, anchor_block)`, where the anchor can be any valid (state, block) pair — not just genesis.

When initializing from a non-genesis anchor:

• The store's justified and finalized checkpoints come from the anchor state
• The head starts at the anchor block
• The store's block and state maps contain only the anchor
• All subsequent blocks build on this anchor

No spec test filler currently tests non-genesis store initialization.

What to test

Write a fork choice filler that:

1. First, builds a chain from genesis to slot 10 (to create a valid mid-chain state)
2. Uses the state and block at slot 10 as the anchor for a new store
3. Builds additional blocks on top of the anchor (slots 11, 12, 13)
4. Verifies the store behaves correctly with the non-genesis anchor

Alternative simpler approach

If the fork choice filler framework supports custom anchor states:

1. Generate a pre-state at a non-zero slot
2. Initialize the store from that state
3. Build blocks on top
4. Verify head, justified, and finalized checkpoints reference the anchor

Key assertions

• `StoreChecks(head_slot=Slot(10))` — initial head is at the anchor
• Justified/finalized checkpoints match the anchor state's checkpoints
• Blocks at slots 11+ are processed correctly on top of the anchor
• Head advances to the new blocks

Where to add the test

Create a new file: `tests/consensus/devnet/fc/test_checkpoint_sync.py`

Code skeleton

"""Checkpoint sync (non-genesis anchor) tests."""

from consensus_testing import (
    BlockSpec,
    BlockStep,
    ForkChoiceTestFiller,
    StoreChecks,
)
from lean_spec.subspecs.containers.slot import Slot


def test_store_from_non_genesis_anchor(
    fork_choice_test: ForkChoiceTestFiller,
) -> None:
    """Store initializes correctly from a non-genesis anchor block and state."""
    # TODO: Check if fork_choice_test supports a custom anchor_state parameter.
    # If not, this test may need framework support to be added first.
    #
    # Alternative: build a long chain first, then use the filler's ability
    # to verify store state at intermediate points.
    pass

Note: This test may require framework support for custom anchor states. Check `ForkChoiceTest.make_fixture` in `packages/testing/src/consensus_testing/test_fixtures/fork_choice.py` to see if a custom anchor is supported. If not, consider opening a companion framework issue.

How to run

uv run fill --fork=devnet --clean -n auto -k test_store_from_non_genesis

References

• `Store.from_anchor`: `src/lean_spec/subspecs/forkchoice/store.py`
• Checkpoint sync concept: the anchor block's `state_root` must match `hash_tree_root(anchor_state)`

[morelucks]

I like to work on this