fix(gate-validator): deterministic tiebreaker for latest-gate sort

When two gate files land in the same millisecond (common on fast CI
filesystems), mtime-only sort is unstable and may pick the wrong
"latest" gate. Add filename localeCompare as a stable secondary sort
so latest-mode validation is reproducible.

Resolves CI flake on tests/gate-validator.test.js:164
"validates every gate when requested".

Author: mumit

scripts/gate-validator.js

@@ -56,7 +56,7 @@ function latestGateFile() {
       const full = path.join(GATES_DIR, name);
       return { name, full, mtime: fs.statSync(full).mtimeMs };
     })
-    .sort((a, b) => b.mtime - a.mtime);
+    .sort((a, b) => b.mtime - a.mtime || b.name.localeCompare(a.name));
 
   return files[0] || null;
 }