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

Both macOS legs are red at one declaration:

projects/ores.workflow/core/include/ores.workflow.core/service/workflow_engine.hpp:383:10:
  error: no type named 'jthread' in namespace 'std'; did you mean 'thread'?

Apple's libc++ has no @c std::jthread. This story first recorded that the stop token and the stop source were available, because the engine's lambda took one and the compiler did not complain: it did not complain because the declaration holding the lambda had already failed. The build after the fix named both as missing too, and the follow-up hotfix implements the stop state in the shim as a result.

The gap has cost a build before: caf6de734d reverted a comms thread to std::thread on 2026-03-12 for the same reason. Reverting this one the same way would leave the next author free to reintroduce it a third time, so the fix is the portable type, in the component that exists for exactly that.

• ores.platform::concurrency::stoppable_thread provides the part of std::jthread the estate uses: it carries a stop token to its callable, and it requests a stop and joins in its destructor.
• The workflow engine uses it, and no source in the estate names std::jthread.
• The tree builds, the workflow and platform suites pass, and both macOS legs link and test.

• The portable type goes in ores.platform, whose concurrency header already says it holds "the types that stand in for a C++20 standard facility a supported standard library does not implement". A revert to raw std::thread in the engine would fix this instance and leave the gap open.
• The callable keeps its std::stop_token signature. The token is the part Apple implements, so the caller's code is unchanged and the portability difference is confined to who owns the thread.
• The type carries no export attribute: every member is defined in the header, and an export attribute on a header-only type is the link failure fixed earlier today in ores.workflow.api.

• The estate's remaining std::thread uses, which are portable already.