Task: Implement Hotfix: Apple's libc++ has no C++20 stop machinery at all

Make the portable thread depend on nothing Apple's libc++ can be missing.

• The header includes only atomic, memory, thread and utility.
• stop_source answers request_stop() and stop_requested(), and copies share one flag.
• The engine's callable takes the shim's and its body is unchanged.
• The tree builds, the workflow and platform suites pass, and a macOS run builds and tests the branch.

1. Read the second macOS diagnostic in full, rather than its first line.
2. Replace the standard stop types with the shim's own, and move the engine.
3. Build, run the suites, format-check, correct the earlier story, and dispatch macOS.

• The first fix's mistake is worth recording plainly: the macOS log named one missing type and the shim was written against that one type. The build after it named two more. The estate's own rule – do not infer a platform gap's boundary from a single diagnostic – is what this task follows.
• iam core's tests are red in the worktree on is_registration_default and account_status, columns main added after the worktree's database was made. They are unrelated to this change and the suites that exercise it pass.

The portable thread carries its own stop source and depends only on thread, atomic, memory and utility. No source names a standard stop type; the tree builds and the workflow and platform suites pass.