async: let an around-task hook tell a resumption from a posted callable

[christianparpart]

StrandOptions::aroundTask / KeyedAroundTask<Key> receive a RunTask that wraps a private StrandTask* and exposes only operator(). A hook therefore cannot tell a coroutine resumption from a posted callable.

Why it matters: morph (LASTRADA-Software/morph#806) installs a Task handler's morph::session through the keyed hook. While a handler is suspended, its session reaches every task on that model's key, including onBackendChanged and a queued action until that action installs its own session. morph documents this. A resumption-only session would be tighter.

Proposal: add [[nodiscard]] bool RunTask::isResumption() const noexcept, or a TaskKind enum, so a hook can apply per-resumption context only to coroutine resumptions.

• Cost: none on the path where no hook is set; one load where a hook is set.
• Tests: the hook observes the kind for a post, a submit(handle) and a submit(ParkedWork).

Not needed for 0.4.x correctness. This is a precision improvement for consumers of the hook.

[christianparpart]

Fixed in v0.5.0 (87e42ed): enum class TaskKind { Callable, Resumption } and RunTask::kind(), derived from StrandTask's existing discriminator (no new field). An enum rather than the proposed bool, per the design rules.