Pulse expects type constructors to be injective

The following is code that works in F*, but not in Pulse:

```fstar
module Natlt
open Pulse.Nolib
#lang-pulse

let natlt (n: nat) = i:nat { i < n }

let works #n (x: natlt n -> bool) (i: nat { i < n }) =
let a: (natlt i -> bool) = x in
()

fn doesnt_work #n (x: natlt n -> bool) (i: nat { i < n }) {
let a: (natlt i -> bool) = x;
()
}
```

Looking at the debugging output, the core type checker produces the guard `forall (_: Natlt.natlt i). i == n` which is obviously not provable.

Start with the Natlt reproduction using Pulse.Nolib and compare the working let example with the failing fn example. Inspect the core type checker’s debugging output around the guard forall (_: Natlt.natlt i). i == n. Done means the fn example type-checks without producing an unprovable guard.

Written by the indexing model from the issue text.