Resolve morph's follow-up from its move onto core::coro

[christianparpart]

morph's coroutine migration (plan Task C8, Step 6) opens a follow-up issue in morph for what it could not move onto core-cpp yet:

• executors and the strand,
• the logger,
• FileIoOps,
• the DateTime clock seam,
• morph::net on Windows.

What is left: resolve that issue, and graduate morph's strand into core::coro once a second consumer needs one (the graduation rule). Link morph's issue here when it exists.

Recorded in .agent/rules/async-and-net.md (Open work). Source: the plan's "Open work after v0.1.0" list in the design plan.

[christianparpart]

Update from the 0.5.0 sweep: the executor/strand part is resolved. core-cpp 0.4.0 gained coroutine-aware core::async::Strand and KeyedStrands (with seal() in 0.4.1), and morph (LASTRADA-Software/morph#806) now drives its model handlers on them instead of its own strand. Still open in morph's lane: the logger, FileIoOps, the DateTime clock seam and morph::net on Windows. Those move only under the graduation rule.