`box::export()` at namespace scope overrides export attribute declarations

### Error description

## MWE

### `mwe.r`

```r
#' @export
test1 = 1

test2 = 2

box::export(test2)
```

```r
box::use(./mwe)
ls(mwe)
```

### Expected result

> ```
> [1] "test1" "test2"
> ```

### Actual result

> ```
> [1] "test2"
> ```

### Additional details

Calling `box::export()` in `.on_load()` works as expected; i.e. when changing `mwe.r` to the following, we get the expected result:

```r
#' @export
test1 = 1

test2 = 2

.on_load = function (ns) {
box::export(test2)
}
```

### R version

```plain text
_
platform aarch64-apple-darwin20
arch aarch64
os darwin20
system aarch64, darwin20
status
major 4
minor 3.1
year 2023
month 06
day 16
svn rev 84548
language R
version.string R version 4.3.1 (2023-06-16)
nickname Beagle Scouts
```

### ‘box’ version

1.1.3

Start by reproducing the MWE in mwe.r with the namespace-scope box::export(test2) call, then compare it with the .on_load() variant. Trace how the @export declaration and box::export() are processed, and add a regression test showing that both test1 and test2 remain exported.

Written by the indexing model from the issue text.