bug: verify checks input/output/proof files before validating the --kernel extension

Packages versions

miden-vm: 0.30.0 (this bug is in miden-vm/src/cli/verify.rs, the CLI's verify command not really version-specific, but this is the current workspace version)

Bug description

The verify command checks for input/output/proof files before validating the --kernel file extension. This is the same kind of bug that was already found and fixed for prove in #3587, just in a different command.

If you run verify with a bad --kernel path (like a .txt file) while the proof file is missing or the input/output files can't be found, you get a confusing error about the proof or input file, instead of being told the actual problem, which is that --kernel has the wrong extension.

I already have a fix ready that checks the kernel extension first, before touching any other file, plus a test. Happy to open a PR if someone assigns this to me.

How can this be reproduced?

1. Run: miden-vm verify --proof missing.proof --kernel bad.txt --program-hash 00
2. If missing.proof doesn't exist, you get "Proof file does not exist" instead of an error about --kernel having the wrong extension.

Relevant log output