CI: gate lint stage on clippy --all-targets -D warnings

Previously the lint job ran fmt --check and clippy separately without
the -D warnings flag, allowing clippy warnings to pass silently.
Now clippy runs with --all-targets (catches issues in tests and
benches) and -D warnings (treats all warnings as errors).

.gitlab-ci.yml

@@ -43,7 +43,7 @@ lint:
   script:
     - rustup component add rustfmt clippy
     - cargo fmt --check
-    - cargo clippy -- -D warnings
+    - cargo clippy --all-targets -- -D warnings
   rules:
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
     - if: $CI_COMMIT_BRANCH