`ssh-keygen` does not generate ed25519 key with pem format

[Darren Rambaud]

Hi -- Using `10.3p1` release:

```
# ssh -V
OpenSSH_10.3p1, OpenSSL 3.6.2 7 Apr 2026
```

I am trying to generate a public/private key pair with this command:

```
ssh-keygen -f ./openssh103p1 -t ed25519 -N '' -m 'PEM'
```

However, it fails to generate a public/private key pair due to:

```
Generating public/private ed25519 key pair.
Saving key "./openssh103p1" failed: invalid format
```

Using nix, I built the latest release, 10.4p1, however result is the same:

```
# ssh -V
OpenSSH_10.4p1, OpenSSL 3.6.2 7 Apr 2026
# ssh-keygen -f ./openssh104p1 -t ed25519 -N '' -m 'PEM'
Generating public/private ed25519 key pair.
Saving key "./openssh104p1" failed: invalid format
```

It works n the `10.2p1` and `10.1p1` (and earlier?) releases:

```
# ssh -V
OpenSSH_10.2p1, OpenSSL 3.6.2 7 Apr 2026
# ssh-keygen -f ./openssh102p1 -t ed25519 -N '' -m 'PEM'
Generating public/private ed25519 key pair.
Your identification has been saved in ./openssh102p1
Your public key has been saved in ./openssh102p1.pub
The key fingerprint is:
SHA256:dFq24FF9/NVePbIKtD9yvJSoMUTjefjm0ilNXr/BMXE darren at machine
The key's randomart image is:
+--[ED25519 256]--+
|    ....
+----[SHA256]-----+

# ssh -V
OpenSSH_10.1p1, OpenSSL 3.6.2 7 Apr 2026
# ssh-keygen -f ./openssh101p1 -t ed25519 -N '' -m 'PEM'
Generating public/private ed25519 key pair.
Your identification has been saved in ./openssh101p1
Your public key has been saved in ./openssh101p1.pub
The key fingerprint is:
SHA256:8yuY3zLEWairs1CVaUcY/imCxfVdZeJ2ru55I02HMEc bldr at bldr
The key's randomart image is:
+--[ED25519 256]--+
|    ...
+----[SHA256]-----+
```

The above results are repeatable across `aarch64-darwin`, `x86_64-linux`, and `aarch64-linux` platforms.

I checked the past 2 release notes and I don't see any indication that this should have changed.

Darren