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.

system/services/fat/engine.zig

@@ -1657,3 +1657,21 @@ test "short-name checksum matches the reference vector" {
     const c = FileSystem.shortChecksum("REDAME  TXT".*);
     try std.testing.expect(a != c);
 }
+
+test "the FAT engine rejects an exFAT volume (mutual exclusion at mount)" {
+    const allocator = std.testing.allocator;
+    const bytes = try allocator.alloc(u8, 5000 * sector_size);
+    defer allocator.free(bytes);
+    @memset(bytes, 0);
+    // An exFAT boot sector: the "EXFAT   " name and 0x55AA, but MustBeZero (offset
+    // 11, where a FAT BPB keeps bytes-per-sector) stays zero — so this engine's
+    // geometryOf reads a zero bytes-per-sector and rejects it.
+    @memcpy(bytes[3..11], "EXFAT   ");
+    bytes[on_disk.boot_signature_offset] = 0x55;
+    bytes[on_disk.boot_signature_offset + 1] = 0xAA;
+    var disk = RamDisk{ .bytes = bytes };
+    try std.testing.expect(FileSystem.mount(disk.device()) == null);
+    // Control: a real FAT16 mounts.
+    formatFat16(bytes);
+    try std.testing.expect(FileSystem.mount(disk.device()) != null);
+}