ci: validate minimum Xcode 16 runtime

[jacklv-coder]

Summary

• add a dedicated Xcode 16.0 / iOS 18.0 SDK CI job
• verify the pinned RootFS with the minimum toolchain
• final-link arm64 Simulator and unsigned-device apps
• install the iOS 18.0 Simulator runtime and run the 13-check native smoke
• pin PocketRoot to IshEmbed v0.4.0-abi.3 and record immutable release evidence
• preserve diagnostic progress, early-exit, unified-log, and crash-report evidence for smoke failures

Validation

• swift test — 119 tests, 1 expected external RootFS skip, 0 failures
• ./Scripts/check-docs.sh
• Tests/Scripts/RuntimeSmokeScriptTests.sh
• local Xcode 26.1.1 / iOS 18.2 final links and 13-check native smoke
• hosted Xcode 16.0 / iOS 18.0 RootFS install, Simulator/device final links, and 13-check native smoke
• GitHub Actions run 30060044409 — both jobs passed
• final Codex CR against origin/main — no actionable correctness issues; P1/P2: 0

Scope

This closes the minimum-Xcode native validation gate. iPad physical-device execution, complete lifecycle/cancellation hardening, PTY work, sustained-load/resource testing, and distribution compliance remain separate gates.