fix: ramdisk ReadDir reports nonexistent paths as errors

ReadDirAt returned 0 (empty listing) for paths that do not exist at
all, so the libc's stat()/access() - which probe directory-ness via
SYS_READDIR - reported every nonexistent path as an existing
directory. Harmless for two years of callers, fatal for GCC's driver:
it access()-tests the optional specs file, got a false positive for
0:/sdk/lib/gcc/x86_64-montauk/14.2.0/specs, then died failing to read
it.

A path now has to have a directory entry or at least one child to
list as a directory; everything else returns -1. Real empty
directories (tar dir entries, runtime Mkdir) still list as empty.

Co-Authored-By: Claude Fable 5 <[email protected]>

kernel/src/Api/BuildNo.hpp

@@ -12,4 +12,4 @@
 
 #pragma once
 
-#define MONTAUK_BUILD_NUMBER 19
+#define MONTAUK_BUILD_NUMBER 20