refactor(fork-choice): enumerate payload key union without copying values (#1137)

Promoting pending aggregate proofs iterated a throwaway merged dict
built only to walk its keys, copying every set value just to discard it.
Enumerate the key union directly with an order-preserving construction.

The new enumeration yields keys in exactly the same order as the old
merged dict: all counted-pool keys in insertion order, then pending-pool
keys not already present, in their insertion order. This preserves the
LMD determinism invariant, where equal-slot ties resolve by first-seen
insertion order. A set union would not preserve that order.

just check passes.

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

Author: tcoratger

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

@@ -869,7 +869,7 @@ 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, **new_payloads}
+            for attestation_data in dict.fromkeys((*known_payloads, *new_payloads))
         }
 
         # Promote into the counted pool and clear the pending one.