ci: retry fmt on stuck_or_timeout_failure

fmt occasionally gets stuck on unstable runners (empty log, 17+ min
with no output). Since clippy depends on fmt, a stuck fmt blocks the
entire critical path: clippy → check-try-runtime → all tests.

retry: max: 1, when: stuck_or_timeout_failure covers runner freeze
without masking actual cargo fmt formatting failures.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitlab-ci.yml

@@ -62,6 +62,9 @@ default:
 
 fmt:
   stage: quick-checks
+  retry:
+    max: 1
+    when: stuck_or_timeout_failure
   script:
     - cargo fmt --all -- --check