Lexer doesn't recognize Int64 minimal value

Writing `Int64` minimal value `-9223372036854775808L` gets you an out of bounds error as the absolute value exceeds the max value of `Int64`.

```sh
> let a = -9223372036854775808L ;;
fatal error: Integer literal 9223372036854775808 exceeds the representable range
| let a = -9223372036854775808L ;;
| ^^^^^^^^^^^^^^^^^^^
|
> let a = -9223372036854775807L - 1L ;;
> a ;;
: Int64
= -9223372036854775808L
>
```

Start by reproducing the reported REPL input `let a = -9223372036854775808L ;;` and inspect the lexer’s integer-literal handling. Compare it with the working `-9223372036854775807L - 1L` form; done means the minimal Int64 literal is accepted and evaluates to `-9223372036854775808L` without the out-of-bounds error.

Written by the indexing model from the issue text.