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

@@ -333,6 +333,7 @@ pub fn build(b: *std.Build) void {
     if (test_case != null) for ([_][]const u8{
         "vfs-test", // the user-space VFS round-trip client
         "fat-test",
+        "exfat-test", // the exFAT mount round-trip client (S4)
         "badge-scope-test", // the guessable-id probe: a second process names the first's node and layer
         "shared-memory-server",
         "shared-memory-client",