Verify pinned Linux XMPP toolchain restoration

Changes: Refresh the environment-access result and document S00-020 restoration, accepted prerequisites and independent QA handoff; preserve probe code and parent dependency graph.
Validation: verify.py passed official HTTPS vendor checksum, Node 24.21.0/npm 11.19.0, fresh-cache strict npm ci and Stanza import. Separate artifact/input/version/strict-setting assertions and tasks.py validate passed. No browser/WSS or product readiness check performed.
