Add new Squirrel.Mac bundle installation method behind flag

Editor assessment

The subject states the feature and its rollout posture completely: a new Squirrel.Mac bundle installation method was added behind a flag. The mechanism, the platform, and the opt-in gating are all named, and the phrase behind a flag honestly signals an experimental change whose risk is contained, which is valuable information in itself. The feat type is right, no scope is attached, and no body is required since the header carries the essentials. No footers appear, and hygiene is clean. A clear message about a cautiously introduced capability.