Pin loop definitions and carry region coordinates through task execution

[ashb]

A task in a loop or mapped region is identified by (region_id, region_index),
but running code, the Execution API and log readers still address it by the
public map_index. If one value served both, a loop pass would look like a mapped
expansion. We derive the public index from the Dag definition pinned to the try
instead of storing it: a task exposes its region index as map_index only when
its pinned definition expands, and -1 otherwise. Ordinary mapped tasks therefore
keep reporting exactly what they did before.

The scheduler and API server must understand loops without importing user code,
so the serialized Dag carries a loop group definition they can read.

The Execution API and the supervisor schema take the region fields as optional,
versioned fields so that older task SDKs keep working against a newer API
server. Lookups of earlier runs refuse loop producers, because an iteration of
one run has no counterpart to resolve in another run.

A log path and a remote log id have to come out the same for the process that
writes the log and for every reader that fetches it later, retired attempts
included, and one loop pass must not overwrite the logs of the pass before it.
Building both from one prepared, batched context gives workers, the triggerer
and log readers the same answer without a query per task. Only loop tasks gain
a pass position; ordinary mapped tasks keep their existing path, so logs already
written stay readable.

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