Consolidate duplicated updater fixtures and test-only plumbing

Maintainer-requested cleanup of the updater and adjacent build/postinstall tests.

Package installation, service lifecycle, Git candidate activation, failure reporting, and build tests repeat fixture setup and preserve test-only injection or forwarding layers. This makes the updater contracts harder to review and maintain.

The cleanup should remove at least 1,000 net lines through shared fixture owners and deletion of unused plumbing, while preserving effective test cases, assertions, platform coverage, timeouts, authority checks, rollback, and cleanup semantics. Any baseline fixture failures should be investigated and recorded; coverage or timeout changes are not a way to meet the line target.

The implementation will include focused validation, measured test costs, independent review, and the normal PR gates. No live deployment is part of this work.