exfat: the service + build wiring (S4 step 4)

exfat.zig is a near-clone of fat.zig — the reuse the architecture
promised, now real: same IpcBlock DMA-bounce wrapper, same
acquire-volume-by-hello to the volume manager, same content-conditional
boot rewrites (resolve /system/configuration to decide the system
volume), all differences confined to the engine it wraps. That the
harness's Server(engine.FileSystem) compiles is the proof the exFAT
engine meets the same pub-fn contract as FAT — a comptime check, not a
hope.

The exfat package (build.zig + build.zig.zon) mirrors fat's: it ships in
production_ship, its on-disk + engine host tests run through the root
test aggregate (replacing the temporary standalone entry), and the root
zon declares it. build + zig build test + bounds all green.

build.zig

@@ -122,6 +122,7 @@ fn driverArtifact(comptime package: []const u8, comptime artifact: []const u8) S
 /// (docs/build-packages-plan.md).
 const production_ship = [_]ShipRow{
     service("fat"),
+    service("exfat"),
     service("display"),
     service("display-demo"),
     service("device-manager"),
@@ -422,10 +423,6 @@ pub fn build(b: *std.Build) void {
         "system/boot-handoff.zig",
         "system/abi.zig",
         "system/initial-ramdisk.zig", // v2 path-named entries: find/basename/magic
-        // The exFAT engine (S4), std-only, before its package exists — the
-        // engine imports on-disk.zig, so this one entry runs both files' tests.
-        // Moves into the `exfat` package's own test step once that lands (step 4).
-        "system/services/exfat/engine.zig",
     }) |root| {
         const mod_tests = b.addTest(.{
             .root_module = b.createModule(.{
@@ -451,6 +448,7 @@ pub fn build(b: *std.Build) void {
         csv_library,
         xkeyboard_config_library,
         b.dependency("fat", .{}),
+        b.dependency("exfat", .{}),
         b.dependency("volume-manager", .{}),
         b.dependency("display", .{}),
         b.dependency("ps2-bus", .{}),