fix(fork-choice): merge aggregate pools in deterministic order (#892)

* fix(fork-choice): merge aggregate pools in deterministic order

The acceptance tick merged the pending and counted aggregate pools by
iterating `known.keys() | new.keys()`. A union of dict-key views is a
plain set, so its iteration order follows hashing, not insertion.

The LMD walk keeps the first-seen vote when two votes share an
attestation slot, so the merged pool's order decides which fork an
equivocating validator counts toward. A hash-ordered merge therefore
made the head depend on the hash seed and diverge across clients —
regressed in #888, surfaced by test_same_slot_equivocating_attesters_count_once
flipping the head from slot 2 to slot 3.

Iterate `{**known, **new}` instead: counted votes first in insertion
order, then pending-only votes, restoring the pre-#888 ordering.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Update src/lean_spec/spec/forks/lstar/fork_choice.py

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tcoratger

src/lean_spec/spec/forks/lstar/fork_choice.py

@@ -830,7 +830,7 @@ def accept_new_attestations(self, store: LstarStore) -> LstarStore:
         merged_aggregated_payloads = {
             attestation_data: known_payloads.get(attestation_data, set())
             | new_payloads.get(attestation_data, set())
-            for attestation_data in known_payloads.keys() | new_payloads.keys()
+            for attestation_data in {**known_payloads, **new_payloads}
         }
 
         # Promote into the counted pool and clear the pending one.