Solve the following problem with Lean 4 code and explanatory comments:

```lean4
import Mathlib


theorem demo (n : ℕ) : n + 0 = n := by sorry
```

Replace every sorry statement with an appropriate proof. Before writing any Lean code, give an informal solution with the sections [Problem Restatement], [Useful Observations], [Reasoning Path], [Key Derivation], [Proof Type], [Formal Anchors] and [Final Solution]. Then provide the complete solution in the lean4 code block. If a previous attempt failed, first give [Error Analysis], [Revised Reasoning] and [Updated Proof Plan], then provide the corrected solution.
