volume-manager: recognize exFAT and route it by content (S4 step 6)

partition.zig gains an exFAT VBR recognizer: the "EXFAT   " name (where a
FAT BPB keeps its OEM string, so the two never collide) plus the 0x55AA
signature, with VolumeSerialNumber (offset 100) as a new exfat_serial
identity rung. A `recognize` helper tries exFAT, then FAT, then the MBR
disk-signature fallback, and sets each volume's FilesystemKind — so
allVolumes/firstVolume and the GPT path all tag a volume with the engine
its content needs. volume-map renders exfat-<serial> as the id-path, and
filesystems.csv adds the exfat -> /system/services/exfat row: an exFAT
stick now spawns the exFAT service, at its own content id-path.

Host tests: a bare exFAT volume recognized with its serial; a FAT VBR
still recognized as fat (the discrimination); the exfat-<serial> id
render. build + zig build test + bounds green.

system/services/volume-manager/volume-map.zig

@@ -42,6 +42,7 @@ pub fn idString(identity: partition.Identity, buf: []u8) []const u8 {
         .gpt_guid => std.fmt.bufPrint(buf, "gpt-{x:0>32}", .{identity.key}) catch "",
         .filesystem_uuid => std.fmt.bufPrint(buf, "uuid-{x:0>32}", .{identity.key}) catch "",
         .fat_serial => std.fmt.bufPrint(buf, "fat-{x:0>8}", .{@as(u32, @truncate(identity.key))}) catch "",
+        .exfat_serial => std.fmt.bufPrint(buf, "exfat-{x:0>8}", .{@as(u32, @truncate(identity.key))}) catch "",
         .mbr_index => std.fmt.bufPrint(buf, "mbr-{x}-{d}", .{
             @as(u32, @truncate(identity.key >> 8)),
             @as(u8, @truncate(identity.key & 0xff)),
@@ -105,6 +106,7 @@ const test_override_slots = 4;
 test "idString renders each rung's id token" {
     var buf: [id_maximum]u8 = undefined;
     try testing.expectEqualStrings("fat-12345678", idString(.{ .rung = .fat_serial, .key = 0x12345678 }, &buf));
+    try testing.expectEqualStrings("exfat-da7a0001", idString(.{ .rung = .exfat_serial, .key = 0xDA7A0001 }, &buf));
     try testing.expectEqualStrings("mbr-deadbeef-1", idString(.{ .rung = .mbr_index, .key = (@as(u128, 0xDEADBEEF) << 8) | 1 }, &buf));
     const guid: u128 = 0x00112233445566778899AABBCCDDEEFF;
     try testing.expectEqualStrings("gpt-00112233445566778899aabbccddeeff", idString(.{ .rung = .gpt_guid, .key = guid }, &buf));