build: Add macOS stub compilation support

[cgwalters]

This is the start of work on #21

For now only the ephemeral verb exists and it just errors out, but the idea is we can fill that in.

Assisted-by: OpenCode (Claude claude-opus-4-5@20251101)

[gemini-code-assist]

Code Review

This pull request introduces stub compilation support for macOS by adding conditional compilation attributes (#[cfg(target_os = "linux")]) to Linux-specific modules and dependencies. This is a good approach to enable cross-platform builds while deferring full feature implementation on other platforms. The changes are well-contained and correctly use Rust's conditional compilation features. I have one suggestion regarding the exit call in the main function to align with idiomatic Rust practices.

[jeckersb]

Needs rebase, but one thing this really wants is something like:

#[cfg(target_os = "linux")]
{
    mod arch;
    mod images;
    // ... etc ...
}

Which isn't valid today. We could do something like move all of the linux-only modules to be submodules under a parent linux module, and then:

#[cfg(target_os = "linux")]
mod linux;
#[cfg(target_os = "linux")]
use linux::*;

But that's a lot of code-motion to re-home everything.

We could also abuse the #[path] attribute, also kind of ugly.

I guess my point is this is kinda ugly but also all of the valid alternatives are also ugly in their own way.

[cgwalters]

The complex thing here is I think at least some of these things will actually start to build on macos (e.g. qemu). So either way - really it's inevitable that we have conditional churn.

Anyways rebased 🏄

[jmarrero]

lgtm