#4382 fix(apple-runner): guard XCTest build cleanup paths

Summary

Use the canonical guarded XCTest cleaner for builds and manual cleanup. Refuse unsafe or symlinked overrides and preserve sibling caches. Print recovery guidance that distinguishes variable-requested cleanup from explicit cleanup, forced rebuilds and cache repair.

Ten files change. Review follow-ups expanded documentation/tests and route cleanup helpers through the existing bound runner facade, removing the private source import and project-root callback. Fixes #4371; part of #4368.

Validation

Head: 0f2338df89eb790c02aadd7bfac34950d7599a7e.

• All 42 runnable exact-head affected checks passed; exactCommit=true, checkoutChanged=false. The full 67-check selection retains 25 owned skips. Related: 1,058; provider: 257 passed; unit: 17,830 passed/one unidentified skip. Receipt and obligations.
• Focused: 18 build fixtures, 20 guard cases, one default-path guard and eight cleanup smoke cases passed; full formatting passed. Independent Luna follow-up review found no concrete regressions.
• Isolated base reproduced unsafe override/stale output. Intermediate symlink regressions failed two/passed 16, then passed 18 with containment. Safe dot-prefixed names reproduced one failure before their fix. Proof.
• Actual iOS Simulator build succeeded at 71957957be with an owned override and removed sentinel. This is historical evidence, not a native/device run at this head.
• Current CI: 16 successful checks and one skipped cache lane; Android, Apple and Linux smoke passed. Apex approves; known review threads are resolved. No merge is authorized.