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.

test/system/services/exfat-test/build.zig.zon

@@ -0,0 +1,14 @@
+.{
+    .name = .exfat_test,
+    .version = "0.0.0",
+    .fingerprint = 0x77b19e3f7ee43ce3, // Changing this has security and trust implications.
+    .minimum_zig_version = "0.16.0",
+    .dependencies = .{
+        // build-support supplies the shared recipe; kernel is implicit in
+        // every binary (the root shim + link script live there). The rest
+        // are exactly the homes of this binary's declared imports.
+        .@"build-support" = .{ .path = "../../../../build-support" },
+        .kernel = .{ .path = "../../../../library/kernel" },
+    },
+    .paths = .{""},
+}