Twelve CI fixes before sunrise — the overnight pipeline that wouldn't go green

Spent 8 PM to 6 AM fixing the CI pipeline — twelve consecutive commits before it finally went green. It started with the xcode-27 runner migration. The macos-26 runners didn't have the Xcode 27 toolchain, so builds failed silently. Switched to xcode-27 runners — now Metal Toolchain wasn't installed. Added it — now test targets compiled for macOS 27 but the CI runner was macOS 26. Lowered deployment targets to 26.0 — Keychain tests failed because CI has no keychain. Marked them .enabled(if:). MLX tests crashed because the @Suite init hit a circular reference. Moved the precondition to module level. Swift Testing setUp() throws wasn't a thing — switched to setUpWithError(). Every | tail pipe was swallowing exit codes and hiding failures. Removed them all. Twelve commits. Zero human interventions. The pipeline finally went green at 5 AM. Nobody writes blog posts about CI — but every refactor after this one shipped with confidence because the pipeline catches regressions in 8 minutes.