Add strict RootFS source download cache

[jacklv-coder]

Summary

• add an optional external download cache for RootFS source-review materialization
• reject repository-local, overlapping, symlinked, missing, oversized, or digest-mismatched cache inputs
• preserve pinned upstream receipt origins and revalidate canonical aports trees
• document the offline workflow in Chinese and English

Why

The current source acquisition manifest may need to be rematerialized when upstream endpoints are temporarily unavailable. A previously verified source-review directory should be reusable as transport input without weakening any source pin or approval gate.

Validation

• ruby -c Scripts/prepare-rootfs-source-bundle.rb
• all RootFS Ruby script suites (77 runs, 335 assertions)
• ./Scripts/check-docs.sh
• ./Scripts/test.sh (139 tests, 1 expected environment-gated skip)
• materialized and independently verified a fresh bundle against the current production manifest
• codex review --uncommitted (no actionable findings)