test: block-range — the discrimination fixture for range confinement (V2a)

A process acquires a block channel the way a filesystem does (consumer-hello
the device manager), confines ITSELF to blocks [1,3), then proves the clamp
and the gate: volume-relative LBA 0 maps inside the range and reads; a read
reaching past the range is refused; geometry reports the confined size; and
a confined caller can no longer call define_range (no widening, no escape).
It gates on argv so the ramdisk sweep leaves it silent in other boots, and
coexists with fat (ranges are per-badge).

Discrimination (verified by reverting usb-storage to pre-clamp f1bdce2~1):
the unconfined read still succeeds but define_range returns ENOSYS, so the
fixture cannot arm confinement and the case fails — exactly the property
the clamp adds. With the clamp: block-range 1/1.

build.zig.zon

@@ -80,6 +80,7 @@
         .@"protocol-denied-test" = .{ .path = "test/system/services/protocol-denied-test", .lazy = true },
         .@"protocol-conformance-test" = .{ .path = "test/system/services/protocol-conformance-test", .lazy = true },
         .@"device-authority-test" = .{ .path = "test/system/services/device-authority-test", .lazy = true },
+        .@"block-range-test" = .{ .path = "test/system/services/block-range-test", .lazy = true },
         // See `zig fetch --save <url>` for a command-line interface for adding dependencies.
         //.example = .{
         //    // When updating this field to a new URL, be sure to delete the corresponding