Formalize Van Eck's sequence coverage conjecture (OEIS A181391)

### What is the conjecture

OEIS A181391 starts with $a(1)=0$. To obtain the next value, look at the current value. If it
has appeared before, write the distance back to its most recent previous occurrence; if it has not
appeared before, write zero.

The sequence begins

$$0,0,1,0,2,0,2,2,1,6,0,5,\ldots$$

OEIS A181391 lists the conjecture:

> every number eventually appears.

Sloane's 2016 open-problems slides also present the coverage statement as unresolved.
The Lean function shifts the source once, so Lean's `a 0` represents OEIS's `a(1)`.

References:

- [OEIS A181391](https://oeis.org/A181391)
- N. J. A. Sloane, [“Some Open Problems”](https://neilsloane.com/doc/EMMay2016.pdf#page=14)

### Prerequisites needed

A primitive-recursive state can store the current value and the most recent prior index of each
value. Mathlib's natural-number arithmetic, `Option`, pointwise function update, and
`Function.Surjective` are sufficient. No addition to `FormalConjecturesForMathlib` is needed.

### [AMS categories](https://github.com/google-deepmind/formal-conjectures/labels?q=ams-)

* ams-11

### Choose either option

- [x] I plan on adding this conjecture to the repository
- [ ] This issue is up for grabs: I would like to see this conjecture added by somebody else

Start by locating the existing Lean function for the shifted sequence and review nearby formalized conjectures. Model the primitive-recursive state using natural-number arithmetic, Option, and pointwise function update, then state the coverage claim with Function.Surjective. Done means the conjecture is added and the project checks it without requiring a new FormalConjecturesForMathlib addition.

Written by the indexing model from the issue text.