A schedule fires dispatch() when the workflow declares one

[czpython]

A subject-backed workflow can't be scheduled: the cron fires run() with no subject, and a run about something needs one. Apps worked around it with a shim workflow whose whole body was Real.dispatch().

Now a schedule fires dispatch() when the class declares one — the policy resolves the subject and starts the real run. The tick runs no workflow of its own kind (no Run row for the tick); dispatch() runs at the scheduled workflow's body level with a session, because DBOS only allows the child-start inside start() from a workflow body — a step is forbidden it (dbos/_context.py asserts is_workflow() on child-start). A failed dispatch fails the .scheduled workflow; the next tick fires fresh.

A scheduled dispatch() fires with no arguments, so declaring one with a required parameter is a class-definition error. Detection is getattr, so a mixin-provided dispatch() schedules the same as a declared one.

The author guide's schedule section now shows the subject-backed shape instead of the shim pattern. The test drives the registered scheduled entry end to end: the tick reaches dispatch(), whose start() enqueues the real run about its subject.

ENG-854