Xcode 26.2+ / Swift 6.2.3+ for all builds

[rgoldberg]

Xcode 26.2+ / Swift 6.2.3+ for all builds.

Waiting for Xcode 26.2 on macos-15 & macos-15-intel GHA runners, which will presumably occur on 2026-01-12 or soon thereafter.

strerror ➡ unsafe strerror