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.zon

@@ -46,6 +46,7 @@
         .@"pci-bus" = .{ .path = "system/drivers/pci-bus" },
         .init = .{ .path = "system/services/init" },
         .fat = .{ .path = "system/services/fat" },
+        .exfat = .{ .path = "system/services/exfat" },
         .display = .{ .path = "system/services/display" },
         .@"display-demo" = .{ .path = "system/services/display-demo" },
         .@"device-manager" = .{ .path = "system/services/device-manager" },