refactor(lstar): trim state-transition docs and use itertools.batched (leanEthereum#1116)

* refactor(lstar): trim state-transition docs and use itertools.batched

Cut the comment volume in the lstar state transition and replace the
flat-vote-list index arithmetic with a stdlib idiom. No behavior change:
the 3SF-mini justification and finalization logic is untouched, and the
state-transition vectors are byte-identical (verified with fill, including
the determinism check across hash seeds).

- Unpack the flat justification vote list with itertools.batched zipped
  against the tracked roots, replacing the i*N:(i+1)*N slicing. The zip is
  strict so the length invariant is enforced, and each batch is listed
  because the per-root tallies are mutated downstream.
- Hoist the validator count, which was recomputed four times.
- Trim over-explained comment blocks to the load-bearing reasoning and one
  sentence per line: the vote-map round trip, the justifiable-slot filter
  (now deferring to the documented rule rather than re-deriving it), the
  finalization no-gap rule, and the finalized-relative flag encoding.
- Glue front-loaded comments to the line they justify, rather than stacking
  them at the top of a block.
- Correct the genesis-anchor comment: genesis is justified and finalized by
  definition as the anchor, not because it "cannot receive votes" (genesis
  self-votes exist; they are dropped by the already-justified filter).

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

* refactor(anchor): inline the single-use head slot lookup

Drop the head_slot local and read store.blocks[store.head].slot directly
in the checkpoint, matching the inline-single-use convention.

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

* refactor(lstar): inline the single-use genesis-parent check

Fold the parent-slot-zero test directly into the if, dropping the
single-use intermediate boolean.

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

---------

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

Author: tcoratger

src/lean_spec/node/anchor.py

@@ -128,13 +128,12 @@ async def from_checkpoint(
 
         # The protocol return type is structural, but only one concrete store ships.
         store = cast(Store, fork.create_store(state, anchor_block, validator_index))
-        head_slot = store.blocks[store.head].slot
 
         return cls(
             validators=state.validators,
             store=store,
             initial_status=Status(
                 finalized=store.latest_finalized,
-                head=Checkpoint(root=store.head, slot=head_slot),
+                head=Checkpoint(root=store.head, slot=store.blocks[store.head].slot),
             ),
         )