exfat: the in-VM mount + mutation drill (S4 step 8)

exfat-volume attaches a bare exFAT data device (serial e0fa0001, from
make-exfat-image.py) beside the FAT boot volume. The volume manager
content-routes it to the exFAT service — not fat — which mounts it at
its id-path /volumes/exfat-e0fa0001; the exfat-test client then reads the
seeded HELLO.TXT and mutates through the mount (mkdir/write/rename/read/
remove). The second engine reusing the shared harness is now proven end
to end, on a real device, in QEMU.

The drill caught what host tests could not: the exfat service was denied
openEndpoint("volume-manager") — it had no protocol.csv grant — so its
hello never reached the manager and its volume never mounted (a silent
spin, no fault). Added the grant mirroring fat's. A new exfatVolumeTest
kernel case boots the tree and spawns the fixture; the run harness grows
an exfat data-volume flavor. Fails against pre-S4 (no exfat binary, csv
row, VBR recognizer, or grant).

system/kernel/tests.zig

@@ -227,6 +227,8 @@ pub fn run(case: []const u8, boot_information: *const BootInformation) void {
         usbStorageTest(boot_information);
     } else if (eql(case, "fat-mount")) {
         fatMountTest(boot_information);
+    } else if (eql(case, "exfat-volume")) {
+        exfatVolumeTest(boot_information);
     } else if (eql(case, "device-list")) {
         deviceListTest(boot_information);
     } else if (eql(case, "pci-scan")) {
@@ -3036,6 +3038,32 @@ fn fatMountTest(boot_information: *const BootInformation) void {
     result();
 }
 
+/// The exFAT mount chain (S4): boot the full tree, which brings up the USB storage
+/// chain. The harness attaches a SECOND device — a data-only exFAT volume — beside
+/// the FAT boot volume, so the volume manager spawns the exfat service for it
+/// (content-routed, its id-path /volumes/exfat-<serial>). Then spawn exfat-test,
+/// which reads the seeded file and mutates through the mount. The reuse of the
+/// shared harness by a second engine is proven end to end here.
+fn exfatVolumeTest(boot_information: *const BootInformation) void {
+    log("DANOS-TEST-BEGIN: exfat-volume\n", .{});
+    if (boot_information.initial_ramdisk_len == 0) {
+        check("bootloader handed over the initial_ramdisk", false);
+        result();
+        return;
+    }
+    const ramdisk = @as([*]const u8, @ptrFromInt(boot_handoff.physicalToVirtual(boot_information.initial_ramdisk_base)))[0..boot_information.initial_ramdisk_len];
+    const rd = initial_ramdisk.Reader.init(ramdisk) orelse {
+        check("initial_ramdisk image is valid", false);
+        result();
+        return;
+    };
+    process.setInitialRamdisk(ramdisk);
+    const init_ok = if (process.spawnBundled("/system/services/init")) true else |_| false;
+    check("init spawned (boots the tree, incl. the volume manager)", init_ok);
+    check("exfat-test client spawned", spawnNamed(rd, "exfat-test"));
+    result();
+}
+
 /// Per-sender range confinement (V2a, docs/volume-manager-plan.md): the fixture
 /// acquires the block channel, confines ITSELF to a sub-range, and asserts it
 /// cannot read past that range or widen it. Boots init in REGISTRY-ONLY mode