refactor(forkchoice): extract AttestationPool from Store

[tcoratger]

Summary

Three parallel dict fields lived directly on Store:

attestation_signatures: dict[AttestationData, set[AttestationSignatureEntry]]
latest_new_aggregated_payloads: dict[AttestationData, set[AggregatedSignatureProof]]
latest_known_aggregated_payloads: dict[AttestationData, set[AggregatedSignatureProof]]

Each was deep-copied with {k: set(v) for k, v in d.items()} in five different Store methods. This PR collapses all three into a single immutable AttestationPool value object that owns the gossip → new → known lifecycle, and rewrites those five methods to delegate to pool methods.

Net diff: store.py shrinks by ~80 lines; one new module (attestation_pool.py); one new test file with 17 cases covering each pool method, including the edge cases flagged as test gaps during research (lone-child early-exit drop, gossip retention by data key).

Pool API

Method	Replaces	Why a method
prune_finalized(slot)	prune_stale_attestation_data body	Encapsulates atomic three-stage prune at finalization advances
add_signature(data, entry)	on_gossip_attestation deep-copy block	Aggregator inbox arrival
add_new_proof(data, proof)	on_gossip_aggregated_attestation deep-copy block	Direct-to-new gossip arrival
add_block_proofs(by_data)	on_block body-attestation merge	Block-included proofs bypass the new stage
migrate_new_to_known()	accept_new_attestations merge step	Slot-rollover stage promotion
replace_after_aggregation(new)	aggregate end-of-method bookkeeping	Wholesale new-stage swap + consumed-signature drop

Read paths stay direct: aggregate, compute_block_weights, update_head, update_safe_target, and produce_block_with_signatures access pool.signatures, pool.new_proofs, pool.known_proofs as plain attributes — every read site already knows which stage it cares about and the docstrings explain why (see e.g. the safe-target rationale).

Field renames on the pool

• attestation_signatures → signatures
• latest_new_aggregated_payloads → new_proofs
• latest_known_aggregated_payloads → known_proofs

The AttestationPool prefix already conveys "attestation"; the latest_ prefix was misleading (the maps don't store latest-vote-per-validator). No back-compat aliases — every call site is updated in this PR.

Behavior preserved exactly

The design phase ran through both consensus-correctness and Pythonic-API reviews before any code was written. Specifically:

• Pruning predicate stays strict-greater (target.slot > finalized_slot); test boundaries verified.
• Aggregation still replaces the new pool wholesale (not a merge): pre-existing entries that did not produce a fresh proof this round still disappear (lone-child early-exit). Gossip retention predicate is keyed on data, not produced-proofs.
• on_block ordering preserved: persist + advance checkpoints → process body attestations → update_head → prune. Pruning runs on the post-update store as before.
• AttestationSignatureEntry stays a NamedTuple (must be hashable for sets) and moves to attestation_pool.py since it has no consumer outside the pool.

Tests

• 17 new unit tests in tests/lean_spec/subspecs/forkchoice/test_attestation_pool.py covering each pool method, including the two edges flagged as gaps in the existing test suite during research:
  • test_replace_after_aggregation_overwrites_new_pool (lone-child entries drop)
  • test_replace_after_aggregation_keeps_unconsumed_signatures (gossip retention)
• Existing tests/lean_spec/subspecs/forkchoice/ and tests/consensus/devnet/fc/ test suites all still pass.
• 3178 total unit tests pass.

Test plan

• uvx tox -e all-checks (ruff, format, ty, codespell, mdformat)
• uv run pytest tests/lean_spec/subspecs/forkchoice/ tests/lean_spec/subspecs/containers/test_state_aggregation.py tests/lean_spec/subspecs/validator/test_service.py — 150 passed
• Full uv run pytest tests/lean_spec --no-cov — 3178 passed
• Reviewer to spot-check replace_after_aggregation semantics against the existing aggregator behavior
• Reviewer to spot-check that no test pierces the renamed fields with stale references

🤖 Generated with Claude Code