Make the release lane replace screenshots instead of adding to them

ios release calls upload_to_app_store without overwrite_screenshots. So deliver skips any screenshot whose checksum is already in App Store Connect, adds the ones that changed, and never deletes the old versions. Once a set reaches Apple's limit of 10, deliver skips the rest ("Too many screenshots found… skipping").

GPS's copy of this lane did exactly that on 2026-09-28 (GPS-14): the iPhone set got duplicates, and the iPad set skipped the new images. Vault hasn't hit it yet, but the next release after a screenshot change would.

Fix: add overwrite_screenshots: true to the release lane's upload_to_app_store, as gps-app #55 did. Every release then replaces the version's screenshots with fastlane/screenshots/en-US, the 12 images make screenshots writes and which are committed. Update RELEASE.md's "Shipping a build" list to say so.