atelet: Add snapshot state tracker and return the latest local or durable snapshot - #2677

)

Part of agent-substrate#2069

ateapi assigns each actor snapshot a generation, but never passed it to
atelet: every `ateletpb.Snapshot` it builds left `generation` unset.
That covers the pause and suspend `Checkpoint`, the paused-actor
`PromoteSnapshot`, and both `Restore` paths. This sets it from the
snapshot each request names, and narrows the TODO on
`Snapshot.generation` to say only that atelet does not check it yet.

Atelet requires generation to be set but doesn't use the value yet. Once
agent-substrate#2677 lands, atelet can reject a `Checkpoint` or `PromoteSnapshot` whose
generation is stale, as the `Snapshot.generation` doc describes. That
check came up in review on agent-substrate#2677.

> It's a good idea to open an issue first for discussion.

- [ ] Tests pass
- [ ] Appropriate changes to documentation are included in the PR

…nd finalizes (agent-substrate#2766)

Part of Actor Lifecycle v2 agent-substrate#2069.

atelet's `Checkpoint` and `PromoteSnapshot` return its snapshot state
for the actor (agent-substrate#2677). The suspend workflow discarded it. This applies
it when a suspend finalizes, with the same rules as a
`ReportActorSnapshotState` report (agent-substrate#2622, agent-substrate#2698).

No proto changes.

- `ensureAteletSuspended` and `ensurePausedSnapshotUploaded` return the
state atelet reports.
- `ensureSuspendedFinalized` applies it with `applyCheckpointState`
before it frees the worker, and records the result in the same update
that commits `SUSPENDED`.
- A durable snapshot records the fidelity atelet reports. For
`Checkpoint` and `PromoteSnapshot`, that's the one requested.
- An upload atelet reports finished for an older generation is marked
`COMPLETED`, so finalize keeps it rather than pruning it as `PENDING`.
- `applyLatestDurable` refuses a reported durable snapshot with no
fidelity, rather than recording `UNSPECIFIED`. atelet echoes the
fidelity it was asked for, and reports already require one.

#### Notes for reviewers

- **The state is never stale.** Every attempt that reaches finalize has
just called atelet, so a retried suspend applies the state from its own
call.
- **A conflicting state fails the suspend.** Finalize applies atelet's
state before it frees the worker. If the state conflicts with the
record, for example a different upload URI, finalize returns the error
and the actor stays `SUSPENDING` with its worker, so a retry can call
atelet again.
- **The lifecycle-op metric's fidelity label** stays the requested
fidelity.
- **With today's atelet, the record doesn't change.** The applied state
matches what finalize already records. It starts to matter once atelet
uploads in the background: an async suspend's response can report an
older snapshot as uploaded before atelet's report arrives, and finalize
then keeps that snapshot as `COMPLETED` instead of pruning it.

- [ ] Tests pass
- [ ] Appropriate changes to documentation are included in the PR