Check scans before slicing

tm/lin_rec.py

@@ -56,6 +56,7 @@ def copy(self) -> HeadTape:
     def to_ptr(self) -> PtrTape:
         return PtrTape(
             sum(int(q.count) for q in self.lspan) - self.head,
+            self.scan,
             self.unroll(),
         )
 
@@ -81,6 +82,7 @@ def step(self, shift: Shift, color: Color, skip: bool) -> int:
 @dataclass
 class PtrTape:
     init: int
+    scan: Color
     tape: list[Color]
 
     def get_ltr(self, start: int) -> TapeSlice:
@@ -354,6 +356,9 @@ def run(self, sim_lim: int) -> Self:  # no-cover
                 if state != init_state:
                     continue
 
+                if tape.scan != init_tape.scan:
+                    continue
+
                 ptr = tape.to_ptr()
 
                 if 0 < (diff := tape.head - init_pos):