refactor(forks): inline single-use get_proposal_head (leanEthereum#1030)

The proposal-head helper had a single caller, was not part of any
contract, and returned (store, store.head) — a redundant tuple whose
second element was derivable from the first.

Inline its three steps (advance time, accept pending attestations, read
the head) directly into block production, keeping the rationale comments.
Drops the now-unused Bytes32 import.

Behavior-preserving: all lstar vectors fill green, byte-identical across
hash seeds.

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

Author: tcoratger

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

@@ -15,29 +15,12 @@
     ValidatorIndex,
 )
 from lean_spec.spec.forks.lstar.errors import RejectionReason, SpecRejectionError
-from lean_spec.spec.ssz import Bytes32, Uint64
+from lean_spec.spec.ssz import Uint64
 
 
 class ValidatorDutiesMixin(LstarSpecBase):
     """Validator duties for the lstar fork."""
 
-    def get_proposal_head(self, store: LstarStore, slot: Slot) -> tuple[LstarStore, Bytes32]:
-        """
-        Get the head for block proposal at given slot.
-
-        Ensures store is up-to-date and processes any pending attestations
-        before returning the canonical head. This guarantees the proposer
-        builds on the most recent view of the chain.
-        """
-        # Advance time to this slot's first interval
-        target_interval = Interval.from_slot(slot)
-        store, _ = self.on_tick(store, target_interval, True)
-
-        # Process any pending attestations before proposal
-        store = self.accept_new_attestations(store)
-
-        return store, store.head
-
     def get_attestation_target(self, store: LstarStore) -> Checkpoint:
         """
         Calculate target checkpoint for validator attestations.
@@ -147,11 +130,15 @@ def produce_block_with_signatures(
                 or if the produced block fails to close a justified divergence
                 between the store and the head chain.
         """
-        # Retrieve parent block.
+        # Build on the freshest canonical head.
         #
-        # The proposal head reflects the latest chain view after processing
-        # all pending attestations. Building on stale state would orphan the block.
-        store, head_root = self.get_proposal_head(store, slot)
+        # Advance time to this slot's first interval, then fold in pending attestations.
+        # The proposal head then reflects the latest chain view.
+        # Building on stale state would orphan the block.
+        target_interval = Interval.from_slot(slot)
+        store, _ = self.on_tick(store, target_interval, True)
+        store = self.accept_new_attestations(store)
+        head_root = store.head
         head_state = store.states[head_root]
 
         # Verify proposer authorization.