test: add cross-platform Go build E2E coverage

[ivanopcode]

Summary

• add an offline vendored mixed Go/script skill fixture
• cover native macOS and Windows install, launch, cache, rollback, recovery, concurrency, repair, and scope behavior
• cover Ubuntu portable behavior and source-aware fail-closed behavior
• authenticate the caller-supplied rc.6 candidate while preserving the committed release-suite pin

Local validation

• native macOS candidate: 16 passed, 4 deselected
• full preserved-pin pytest: 2317 passed, 75 skipped
• strict mypy: passed
• build and Twine: passed
• staged diff and release-boundary checks: passed

Task: TASK-260720-3pemm6