Correct coverage cache comment and shard docs

Review follow-ups: rust-cache stores dependencies only, so shard 1's
save does not depend on which targets it built; the shard script's
extra arguments now reach `cargo llvm-cov` too; the coverage-mode test
no longer reads CI_TEST_TIMINGS from the caller's environment.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0174QivDXDqjSDhKth66oLF4

Author: claude

.github/workflows/ci.yml

@@ -626,8 +626,8 @@ jobs:
       - name: Cache cargo
         # Swatinem/rust-cache, main-only saves: see the first `Cache cargo`
         # step in this file.
-        # The shards share one cache key; shard 1 alone writes it, from the
-        # warm step that builds every target.
+        # The shards share one cache key and shard 1 alone writes it.
+        # rust-cache keeps only dependencies, which every shard builds.
         uses: Swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4 # v2.9.1
         with:
           save-if: ${{ github.ref == 'refs/heads/main' && matrix.shard == 1 }}

scripts/ci-test-shard.py

@@ -19,7 +19,7 @@
 no doctests, which cargo-llvm-cov leaves out without `--doctests`. A later
 `cargo llvm-cov report` exports the shard's profiles.
 
-Extra arguments after `SHARD COUNT` go to every `cargo test` invocation.
+Extra arguments after `SHARD COUNT` go to every `cargo` invocation.
 Each invocation runs with `--no-fail-fast`; the exit status is non-zero if
 any of them failed.
 

scripts/tests/test_ci_test_shard.py

@@ -86,7 +86,8 @@ def test_coverage_shards_run_llvm_cov_without_doctests(self):
     def test_coverage_mode_comes_from_the_environment(self):
         metadata = {"workspace_members": ["a"], "packages": [
             {"id": "a", "targets": [{"name": "integration", "kind": ["test"]}]}]}
-        with patch.dict(shard.os.environ, {"CI_TEST_COVERAGE": "1"}), \
+        env = {k: v for k, v in shard.os.environ.items() if k != "CI_TEST_TIMINGS"}
+        with patch.dict(shard.os.environ, {**env, "CI_TEST_COVERAGE": "1"}, clear=True), \
                 patch.object(shard.subprocess, "run", side_effect=[
                     subprocess.CompletedProcess([], 0, stdout=json.dumps(metadata)),
                     subprocess.CompletedProcess([], 0),