fetch: add `--all-platforms` to fetch every variant

- Iterate the full OS/arch matrix plus each `language` for
  `--all-platforms`, downloading each distinct URL once. This covers
  per-macOS-version URL variants, not just a fixed set of corners.
- Collapse to a single combination for casks without `on_system`
  blocks, since they resolve identically on every platform.
- Tolerate `CaskInvalidError`/`CaskUnreadableError` for casks that omit
  some platforms (mirrors the `bump-cask-pr.rb` rescue pattern).
- Warn when a cask is loaded from the API, since OS/arch cannot be
  simulated without the source.

Signed-off-by: Patrick Linnane <patrick@linnane.io>