Function decide_fs 

pub fn decide_fs(
    tier: SandboxPolicy,
    os_enabled: Option<bool>,
    fs_available: bool,
    escalation: SandboxEscalation,
    approval: Option<&dyn PermissionsApprovalHandler>,
    subject: &str,
) -> FsDecision

Decide what to do about FILESYSTEM confinement for one subprocess spawn. Pure — fs_available is the caller’s REAL probe result (landlock_available on Linux, true on macOS via the existing seatbelt path which this function is not consulted for — see tools::builtins::build_sandboxed_sh’s doc comment), never computed internally, so every branch (including the platform-can’t-enforce ones) is directly testable without touching a kernel.