reorg: move test fixtures to test/system/services (source + boot volume)

The 11 QEMU-suite fixtures lived mixed into system/services/ with their
binaries bundled at /system/tests/<name>. Now the repo path is the boot
path, like every real service: test/system/services/<name>. fat-test
moves out of the fat server's directory into its own; display-demo
stays a boot service.

- kernel VFS: setInitialRamdisk derives one read-only initrd mount per
  top-level tree named by the ramdisk entry paths (/system, /test),
  registers ancestors generically with self-parented roots, and refuses
  backend shadowing of any initrd tree
- EFI loader: the fallback walk also enumerates \test (optional — a
  volume without fixtures still boots); manifest and capsule unchanged
- path literals: vfs-test self-open + create probe, process-test
  process_enumerate matches, the args-echo argv[0] expectation; the
  kvfs case now covers the /test root end to end
- docs: DFHS /test rows, tree diagrams, loader prose, and the location
  convention gain the third home; fixed the input-source link

100/100 QEMU cases pass.

system/kernel/log-ring.zig

@@ -187,7 +187,7 @@ test "wrap reclaims whole records and keeps tail on a boundary" {
     while (i < 200) : (i += 1) {
         var message: [64]u8 = undefined;
         const m = std.fmt.bufPrint(&message, "line {d} padding padding padding", .{i}) catch unreachable;
-        _ = ring.append(1, "/system/tests/writer", .info, i, m, false);
+        _ = ring.append(1, "/test/system/services/writer", .info, i, m, false);
     }
     try std.testing.expect(ring.head - ring.tail <= 4096);