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

kernel/src/Fs/Ramdisk.cpp

@@ -280,6 +280,14 @@ namespace Fs::Ramdisk {
             outNames[count++] = entryName;
         }
 
+        // A path with no directory entry and no children does not exist.
+        // Without this, ReadDir on a nonexistent path returned 0 (empty),
+        // which userspace stat()/access() misread as "directory exists" -
+        // fatal for GCC's driver, which access()-tests optional spec files.
+        if (count == 0 && seen == 0 && pathLen > 0 && selfIdx < 0) {
+            return -1;
+        }
+
         return count;
     }