style(slot): use future annotations and unquote self-references (#1140)

The slot module is not a Pydantic model: Slot is a plain Uint64 (int)
subclass, so the project prohibition on future annotations in Pydantic
files does not apply here.

Add the future import and unquote the now-unnecessary self-referential
annotations. Behavior is identical; just check passes.

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

Author: tcoratger

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

@@ -1,5 +1,7 @@
 """Slot primitive shared by the consensus and crypto layers."""
 
+from __future__ import annotations
+
 import math
 from typing import Final
 
@@ -12,7 +14,7 @@
 class Slot(Uint64):
     """A slot number, as a 64-bit unsigned integer."""
 
-    def justified_index_after(self, finalized_slot: "Slot") -> int | None:
+    def justified_index_after(self, finalized_slot: Slot) -> int | None:
         """
         Return the relative bitfield index for justification tracking.
 
@@ -25,7 +27,7 @@ def justified_index_after(self, finalized_slot: "Slot") -> int | None:
         # Slot (finalized_slot + 1) maps to index 0.
         return int(self - finalized_slot) - 1
 
-    def is_justifiable_after(self, finalized_slot: "Slot") -> bool:
+    def is_justifiable_after(self, finalized_slot: Slot) -> bool:
         """
         Whether this slot is a valid justification candidate after a given finalized slot.