Task: Implement Hotfix: the workflow deadline watch uses a thread Apple's libc++ lacks

Make the deadline watch portable, so both macOS legs build and test again.

• stoppable_thread starts a thread with a token, requests a stop and joins in its destructor, and is movable and not copyable.
• The workflow engine declares and constructs it, and grep for std::jthread over projects/ finds nothing.
• The tree builds; the workflow, platform and iam core suites pass.
• A macOS run builds and tests the branch.

1. Write the type, with a comment saying why it exists and why it is not exported.
2. Add it to the concurrency aggregate header and to the component file list.
3. Read the engine's two sites and move them onto it.
4. Build, run the suites, format-check, and dispatch macOS.

• The first draft initialised the thread in the constructor's member-init list and read the stop source there. clang flags that under -Werror as an uninitialised field read, and the fix is to take the token and assign the thread in the body: the token is then a plain value and the initialisation order cannot be argued about.
• The macOS run of the previous fix (36704140032) succeeded, so the failure this task fixes is the one the deadline watch introduced afterwards.

ores.platform::concurrency::stoppable_thread carries the stop token and joins in its destructor, and the deadline watch uses it. No source in the estate names std::jthread; the six affected suites pass and the tree builds.