test: the attacker the device suite never had

The audit's sharpest finding was structural, not a bug: a fully green suite
had hidden six real defects because it contains no attacker. Every device
case asserts that a driver handed its own hardware can drive it. None asked
what a process handed NOTHING can do.

device-authority-test is that process. It is spawned with no device and
asserts what it therefore cannot do: it cannot give away a device another
task holds, nor a free one, because the kernel's rule is that you may give
away what you hold and the device's state is irrelevant to a process holding
nothing. Asserted across every device the machine actually has, so it cannot
pass by accident of which one happened to be free at boot — six on QEMU,
none of them its.

A positive control runs first. device_enumerate works from this process, so
the refusals below it are decisions rather than a syscall path that is
simply broken here; without it, "everything failed" would read identically
to "the assertions are meaningless". A nonexistent device is refused as
NoSuchDevice rather than NotHeld, because a refusal that cannot name its own
rule is what cost a debugging session on the Ryzen.

What it deliberately does not assert, and says so in its header:
device_claim is still first-come-first-served at this point in the run. That
is the hole D6 closes, and the claim half of the invariant joins this
fixture then. Asserting it now would be writing a test that documents the
bug.

Verified to discriminate: removing the holder check flips "every transfer by
a non-holder is refused" while the positive control keeps passing.

Suite 117 -> 118.

test/system/services/device-authority-test/build.zig

@@ -0,0 +1,15 @@
+//! The device-authority-test fixture as a binary package (docs/build-packages-plan.md):
+//! this file names the binary and EXACTLY the modules its source imports —
+//! build-support resolves each name from the domains this zon declares.
+
+const std = @import("std");
+const build_support = @import("build-support");
+
+pub fn build(b: *std.Build) void {
+    const exe = build_support.userBinary(b, .{
+        .name = "device-authority-test",
+        .root_source_file = b.path("device-authority-test.zig"),
+        .imports = &.{ "driver", "logging", "process" },
+    });
+    b.installArtifact(exe);
+}