chore: use JUSTIFICATION_LOOKBACK_SLOTS (leanEthereum#131)

* refactor: use JUSTIFICATION_LOOKBACK_SLOTS

* fix: docs

Author: unnawut

src/lean_spec/subspecs/forkchoice/store.py

@@ -16,6 +16,7 @@
 
 from lean_spec.subspecs.chain.config import (
     INTERVALS_PER_SLOT,
+    JUSTIFICATION_LOOKBACK_SLOTS,
     SECONDS_PER_INTERVAL,
     SECONDS_PER_SLOT,
 )
@@ -725,7 +726,8 @@ def get_attestation_target(self) -> Checkpoint:
         ensuring the target is in a justifiable slot range:
 
         1. **Start at Head**: Begin with the current head block
-        2. **Walk Toward Safe**: Move backward (up to 3 steps) if safe target is newer
+        2. **Walk Toward Safe**: Move backward (up to `JUSTIFICATION_LOOKBACK_SLOTS` steps)
+           if safe target is newer
         3. **Ensure Justifiable**: Continue walking back until slot is justifiable
         4. **Return Checkpoint**: Create checkpoint from selected block
 
@@ -745,11 +747,11 @@ def get_attestation_target(self) -> Checkpoint:
         # Start from current head
         target_block_root = self.head
 
-        # Walk back toward safe target (up to 3 steps)
+        # Walk back toward safe target (up to `JUSTIFICATION_LOOKBACK_SLOTS` steps)
         #
         # This ensures the target doesn't advance too far ahead of safe target,
         # providing a balance between liveness and safety.
-        for _ in range(3):
+        for _ in range(JUSTIFICATION_LOOKBACK_SLOTS):
             if self.blocks[target_block_root].slot > self.blocks[self.safe_target].slot:
                 target_block_root = self.blocks[target_block_root].parent_root
 

tests/consensus/devnet/fc/test_attestation_target_selection.py

@@ -313,7 +313,7 @@ def test_attestation_target_justifiable_constraint(
     Justifiability Rules (see Slot.is_justifiable_after)
     -----------------------------------------------------
 
-    The target starts from current head and walks back at most 3 slots towards safe target.
+    The target starts from current head and looks back at most 3 slots towards safe target.
 
     Then, a slot is deemed justifiable at distance delta from finalization if:
     1. delta ≤ 5