testgen-hs executable probe accepts any runnable binary

[ginnun]

In crates/node/src/cbor/fallback_decoder.rs (and after #537 merges, in crates/testgen/src/testgen.rs), the helper that looks up testgen-hs on disk validates candidates like this:

fn is_our_executable(path: &Path) -> bool {
    Command::new(path).arg("--version").output().is_ok()
}

.is_ok() only means the child could be spawned and its output captured, a non-zero exit code or unrelated --version text is still accepted. An unrelated binary named testgen-hs on PATH, or a broken build, can pass this check.

The same pattern exists in crates/common/src/find_libexec.rs:77 (already carrying a TODO comment there).

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