refactor(forks/lstar): make the genesis anchor slot explicit (leanEthereum#808)

Anchoring the genesis block as justified and finalized swapped in the
parent root but kept the pre-state checkpoint slot. That was correct only
because both checkpoint slots are still zero when the first post-genesis
block is processed, a hidden dependency a reader had to reconstruct.

Write the slot as zero directly at both the header-processing anchor and
its mirror in the block builder, so the genesis anchor reads as a fixed
slot-zero checkpoint. No behavior change.

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

Author: tcoratger

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

@@ -243,8 +243,8 @@ def process_block_header(self, state: State, block: Block) -> State:
         #   updates rely entirely on validator attestations which are processed
         #   later in the block body.
         if is_genesis_parent:
-            state.latest_justified = Checkpoint(slot=state.latest_justified.slot, root=parent_root)
-            state.latest_finalized = Checkpoint(slot=state.latest_finalized.slot, root=parent_root)
+            state.latest_justified = Checkpoint(slot=Slot(0), root=parent_root)
+            state.latest_finalized = Checkpoint(slot=Slot(0), root=parent_root)
 
         # Historical Data Management
 
@@ -644,7 +644,7 @@ def build_block(
             # updates the justified root to parent_root. Apply the same
             # derivation here so attestation sources match.
             if state.latest_block_header.slot == Slot(0):
-                current_justified = Checkpoint(slot=state.latest_justified.slot, root=parent_root)
+                current_justified = Checkpoint(slot=Slot(0), root=parent_root)
             else:
                 current_justified = state.latest_justified