fix: add Nat.cast normalizer missing case

This PR adds a missing `Nat.cast` missing normalization rule for `grind`.
Example:
```lean
example (n : Nat) : Nat.cast n = n := by
  grind
```

src/Init/Grind/Norm.lean

@@ -82,6 +82,7 @@ theorem natCast_mod (a b : Nat) : (NatCast.natCast (a % b) : Int) = (NatCast.nat
 theorem natCast_add (a b : Nat) : (NatCast.natCast (a + b : Nat) : Int) = (NatCast.natCast a : Int) + (NatCast.natCast b : Int) := rfl
 theorem natCast_mul (a b : Nat) : (NatCast.natCast (a * b : Nat) : Int) = (NatCast.natCast a : Int) * (NatCast.natCast b : Int) := rfl
 theorem natCast_pow (a b : Nat) : (NatCast.natCast (a ^ b : Nat) : Int) = (NatCast.natCast a : Int) ^ b := by simp
+theorem natCast_id (a : Nat) : NatCast.natCast a = a := rfl
 
 theorem Nat.pow_one (a : Nat) : a ^ 1 = a := by
   simp
@@ -184,7 +185,7 @@ init_grind_norm
   Int.ediv_zero Int.emod_zero
   Int.ediv_one Int.emod_one
   Int.negSucc_eq
-  natCast_div natCast_mod
+  natCast_div natCast_mod natCast_id
   natCast_add natCast_mul natCast_pow
   Int.one_pow
   Int.pow_zero Int.pow_one

tests/lean/run/grind_natCast.lean

@@ -5,3 +5,9 @@ example (x : Nat) : NatCast.natCast x ≥ 0 := by grind
 example (x : Nat) : x ≥ (-1 : Int) := by grind
 example (x : Nat) : Int.ofNat x ≥ (-1 : Int) := by grind
 example (x : Nat) : NatCast.natCast x ≥ -1 := by grind
+
+example (n : Nat) : Nat.cast n = n := by
+  grind
+
+example (n m a : Nat) : n = m → Nat.cast n - a = m - a := by
+  grind