exfat: cross-engine discrimination + the exfat-test fixture (S4 step 7)

Each engine now refuses the other's volume at mount(): the exFAT engine
rejects a FAT boot sector (a non-zero byte where exFAT keeps MustBeZero),
and the FAT engine rejects an exFAT one (MustBeZero reads as a zero
bytes-per-sector), each mounting its own as a control. The two engines
can never claim the same medium.

exfat-test is the mount round-trip client, cloned from fat-test: it waits
for /volumes/exfat-e0fa0001, reads the seeded HELLO.TXT, then exercises
mkdir + write + rename + read-back + remove through the VFS and reports
"exfat-test: ok". It ships as a lazy fixture in test builds (the
exfat-volume QEMU case, step 8, drives it). build + zig build test +
bounds green.

build.zig.zon

@@ -65,6 +65,7 @@
         .@"virtio-gpu" = .{ .path = "system/drivers/virtio-gpu" },
         .@"vfs-test" = .{ .path = "test/system/services/vfs-test", .lazy = true },
         .@"fat-test" = .{ .path = "test/system/services/fat-test", .lazy = true },
+        .@"exfat-test" = .{ .path = "test/system/services/exfat-test", .lazy = true },
         .@"badge-scope-test" = .{ .path = "test/system/services/badge-scope-test", .lazy = true },
         .@"shared-memory-server" = .{ .path = "test/system/services/shared-memory-server", .lazy = true },
         .@"shared-memory-client" = .{ .path = "test/system/services/shared-memory-client", .lazy = true },