ADFA-2602: Candidate variant of release.yml for the toolchain upgrade

Disposable branch, never merged. Reuses the registered release.yml name so
workflow_dispatch on this branch runs this copy. Differences from stock:

- no merge_stage_to_main job; builds the dispatched branch, not main
- assets come from the adfa-2602-rc draft release on this repo instead of
  the live dev-assets server (which still hosts the 8.14.3 toolchain)
- documentation.db.br comes from the same draft release, not Google Drive
- R2 upload and Slack are gated behind a publish input (default off);
  Firebase App Distribution to testers stays on

Claude-Session: https://claude.ai/code/session_017HGpMsUzZ5wxCfMtDZ2HGP

Author: alome007