test(TASK-047): split the ntfs short-name guard test by platform reality

On NTFS the fixture self-collides: .builtin-manifest auto-generates the
8.3 short name BUILTI~1, so writing that fixture file opens the manifest
through its alias and clobbers it -- the exact threat the guard models.
Gate the fixture-based test to unix, and add a windows counterpart that
proves the collision where 8.3 generation is enabled and asserts the
filter invariant (alias-shaped entries are neither removal candidates
nor claimed) either way.

src/config/builtin_manifest.rs

@@ -320,6 +320,15 @@ mod tests {
         assert!(!is_ntfs_short_name_alias(".builtin-manifest"));
     }
 
+    /// This fixture is unrealizable on NTFS: `.builtin-manifest` gets an
+    /// auto-generated 8.3 short name of exactly this shape (`BUILTI~1`), so
+    /// writing `dir/BUILTI~1` opens the manifest through its alias and
+    /// clobbers it instead of creating a distinct file — the precise
+    /// collision [`is_ntfs_short_name_alias`] defends against. Only
+    /// filesystems without 8.3 aliasing can host alias-shaped names as real,
+    /// distinct files; Windows coverage lives in
+    /// `ntfs_short_name_entries_never_accepted_on_windows` below.
+    #[cfg(unix)]
     #[test]
     fn ntfs_short_name_entries_never_direct_deletions() {
         let dir = fresh_dir("builtin-manifest-shortname-");
@@ -335,4 +344,55 @@ mod tests {
         );
         let _ = fs::remove_dir_all(&dir);
     }
+
+    /// Windows counterpart of `ntfs_short_name_entries_never_direct_deletions`:
+    /// alias-shaped files cannot exist as distinct fixtures here, so the guard
+    /// invariant is proven through the manifest filter instead. Only entries
+    /// accepted into the previous-manifest set can become removal candidates
+    /// (`previous.difference(shipped)`) or be claimed as owned
+    /// (`shipped ∩ previous`); listing the alias names in `shipped` makes the
+    /// second path observable in the rewritten manifest — a broken guard
+    /// would claim them. Where the volume generates 8.3 short names, the
+    /// self-collision that makes the unix fixture unrealizable is also
+    /// asserted directly; where generation is disabled, the alias simply
+    /// never materializes and the invariant assertions stand on their own.
+    #[cfg(windows)]
+    #[test]
+    fn ntfs_short_name_entries_never_accepted_on_windows() {
+        let dir = fresh_dir("builtin-manifest-shortname-win-");
+        fs::write(
+            dir.join(BUILTIN_MANIFEST_FILE),
+            "BUILTI~1\nbuilti~1.sh\nkept.sh\n",
+        )
+        .unwrap();
+        fs::write(dir.join("kept.sh"), "current").unwrap();
+
+        let alias = dir.join("BUILTI~1");
+        if alias.exists() {
+            assert_eq!(
+                fs::read_to_string(&alias).unwrap(),
+                fs::read_to_string(dir.join(BUILTIN_MANIFEST_FILE)).unwrap(),
+                "the 8.3 alias opens the manifest itself, not a distinct file"
+            );
+        }
+
+        reconcile_builtin_dir(
+            &dir,
+            &shipped(&["BUILTI~1", "builti~1.sh", "kept.sh"]),
+            &shipped(&[]),
+        )
+        .unwrap();
+
+        assert!(dir.join("kept.sh").exists(), "real shipped file untouched");
+        assert!(
+            dir.join(BUILTIN_MANIFEST_FILE).exists(),
+            "manifest survives reconciliation"
+        );
+        assert_eq!(
+            fs::read_to_string(dir.join(BUILTIN_MANIFEST_FILE)).unwrap(),
+            "kept.sh\n",
+            "alias-shaped entries are never accepted: neither removal candidates nor claimed"
+        );
+        let _ = fs::remove_dir_all(&dir);
+    }
 }