fix: compile on Linux, and check that it does

[pathscale]

This crate does not build on Linux:

2656 |  callback(RawWindow { _marker: &marker });
     |           ^^^^^^^^^ missing `default_vbox` and `gtk_window`
error: could not compile `tauri-runtime-blitz` (lib) due to 4 previous errors

RawWindow is Tauri's escape hatch to the underlying native window, and on Linux and the BSDs its shape is a gtk::ApplicationWindow and a gtk::Box. This runtime draws with Blitz and owns no GTK window, so those fields cannot be filled with anything true.

The callback is skipped on those platforms. An embedder reaching for a GTK handle from a Blitz window is asking for something that does not exist, and it is better that its integration does not run than that it is handed a window belonging to something else.

Why CI did not catch it

The job named Linux never compiled anything for Linux. cargo tree resolves without building, and cargo package --no-verify skips the build by definition. So the crate stopped compiling there while every check stayed green, and it surfaced in a consumer instead, as four errors inside a dependency nobody had edited.

It now runs cargo check -p tauri-runtime-blitz --features agent-control, with the GTK headers that needs. That step is the point of this PR as much as the fix is.

How it was found

qa-inspect-host depends on this crate, so ps-qa could not be run on a Linux runner at all — which is what pathscale/ps-blitz#86 needs to drive its fixtures.

macOS is unaffected: cargo check --all-features is clean. I have no Linux machine here, so the new CI step is the verification.

[pathscale]

Superseded by #51, which fixes the cause rather than the symptom.

This PR made the window runtime compile on Linux. But a headless QA host opens no window and should never have been compiling that runtime in the first place, so the Linux build of it was never on the path that mattered. It also added GTK development headers to CI, which is exactly the wrong direction for a headless test.

#51 moves the inspection and activation surface out of runtime.rs, makes tauri optional, and leaves a headless consumer with zero window-stack crates in its graph.