chore(#3331): promote local/no-elapsed-assertion warn->error

#3314 delivered the precondition (ADR-456 §(a) reachability-based
clock-control rule + deterministic backfill for commands.cts/init.cts/
io.cts). Current tests/**/*.cjs corpus has zero violations, verified
via `npx eslint 'tests/**/*.cjs' --rule '{"local/no-elapsed-assertion":"error"}'`
before flipping the config, so no fix/delete-and-replace work was needed.

eslint.config.mjs

@@ -450,7 +450,9 @@ export default tseslint.config(
       'no-only-tests/no-only-tests': 'error',
       // Timing anti-patterns — ratcheted to error after cleanup (all violations fixed)
       'local/no-magic-sleep-in-tests': 'error',
-      'local/no-elapsed-assertion': 'warn',
+      // Promoted warn->error by #3331 once #3314 delivered its precondition (ADR-456 §(a) amended,
+      // direct-use modules backfilled with deterministic time control) — see TESTING-STANDARDS.md.
+      'local/no-elapsed-assertion': 'error',
       // Ban raw fs.rmSync in tests — use helpers.cleanup() for Windows-EBUSY retry budget
       'local/no-raw-rmsync-in-tests': 'error',
       // Ban tautological assertions (always-truthy arg or identical-literal equality)