ci: read the unit workers' JUnit reports when the red-main coordinator attributes a failure #3324

Part of #2934. Found on the first live red main, incident #3315.

Problem

For a failed unit shard, the red-main coordinator reads unit-report.xml from the unit-proof-<n> artifact (runFailures, scripts/red-main-coordinator.ts:75-82). The hosted unit jobs never write a file by that name. The artifact holds one report per worker, tmp/unit-worker-<k>-report.xml. So the coordinator finds no report, and every unit failure is recorded with test: null. Per-test classification never happens: a prior suspect, the closure check on a single test, and the flake probe of one file.

Evidence: main run 41488767631. unit-proof-1 contains unit-worker-1-report.xml and unit-worker-2-report.xml. Given the second file, junitFailures returns [{"test":"packages/ui-react/tests/artifact-recovery-runtime.test.ts","signature":"assertion"}]. The record still reads {"test":null,"category":"unit",...}.

The coordinator tests passed because their fake artifact used the assumed name.

Acceptance criteria

• The coordinator reads every unit-worker-<k>-report.xml in the shard's artifact, and still reads unit-report.xml if a job writes one.
• A coordinator test builds its fake artifact from the real layout (tmp/unit-worker-1-report.xml, tmp/unit-worker-2-report.xml) and asserts that the failing test file is recorded.
• Mutation receipt: restoring the single unit-report.xml lookup fails that test.