The value and readonly attribute of a variable can be overriden by getopts, read, the for loop and the `${name=word}` expansion

I have not observed this unique, non POSIX-compliant, behavior in ANY other shell:

```console
$ readonly a
$ readonly -p
readonly PWD=...
readonly a=''
$ read a < 1
> EOF
$ echo "$a"
1
$ readonly -p
readonly PWD=...
$ readonly a
$ readonly -p
readonly PWD=...
readonly a=''
$ for a in 1; do echo "$a"; done
1
$ readonly -p
readonly PWD=...
$ # Likewise for getopts and ${a:=word}
```

If a readonly variable can be trivially overridden like this, it is useless.

Relevant POSIX reading:

[2.5.3 Shell Variables](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_05_03):

> New variables can be defined and initialized with variable assignments, with the [read](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/read.html) or [getopts](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/getopts.html) utilities, with the name parameter in a for loop, with the ${name=word} expansion, or with other mechanisms provided as implementation extensions.

[readonly — DESCRIPTION](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_24_03)

> The values of variables with the [readonly](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#readonly) attribute cannot be changed by subsequent assignment or use of the [export](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#export), [getopts](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/getopts.html), [readonly](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#readonly), or [read](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/read.html) utilities, nor can those variables be unset by the [unset](https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#unset) utility.

(The for loop is left out of the discussion, so maybe it is exempted?)