Add `to_actor` one-shot parallelism example

Demo both flavors of the new API in a runnable script
(auto-collected by `test_docs_examples.py`),

- the fully-implicit one-shot which boots (and tears down) the
  actor-runtime around a single `to_actor.run()` call,
- the concurrent "worker-pool-ish" prime-check pattern: a local
  `trio` task nursery scheduling one-shots against a shared
  caller-managed `an`, mirroring (in miniature) the neighboring
  `concurrent_actors_primes.py` example per issue #477.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

examples/parallelism/to_actor_one_shots.py

@@ -0,0 +1,83 @@
+'''
+`tractor.to_actor.run()`: one-shot single-task subactor
+invocation, the SC-parallelism sibling of
+`trio.to_thread.run_sync()` (and `anyio.to_process`).
+
+Each call spawns a subactor, schedules the async fn as
+its lone remote task, waits on the result and reaps the
+subactor. Concurrency composes the plain `trio` way:
+schedule multiple one-shot calls in a local task nursery
+against a shared actor-nursery; any remote error raises
+directly in the task which scheduled it.
+
+'''
+import math
+
+import tractor
+import trio
+
+
+async def is_prime(
+    n: int,
+) -> bool:
+    if n < 2:
+        return False
+    if n == 2:
+        return True
+    if n % 2 == 0:
+        return False
+
+    sqrt_n = int(math.floor(math.sqrt(n)))
+    for i in range(3, sqrt_n + 1, 2):
+        if n % i == 0:
+            return False
+    return True
+
+
+async def main() -> None:
+
+    # fully implicit one-shot: boots the actor-runtime,
+    # spawns a subactor, runs the task, reaps the
+    # subactor, tears the runtime back down.
+    assert await tractor.to_actor.run(
+        is_prime,
+        n=2,
+    )
+
+    # the "worker-pool-ish" pattern from the original
+    # `concurrent.futures` example: one subactor per
+    # input, all concurrent, results and errors
+    # collected by caller-side tasks.
+    results: dict[int, bool] = {}
+
+    async def check(
+        an: tractor.ActorNursery,
+        n: int,
+        i: int,
+    ) -> None:
+        results[n] = await tractor.to_actor.run(
+            is_prime,
+            an=an,
+            name=f'prime_checker_{i}',
+            n=n,
+        )
+
+    inputs: list[int] = [
+        7,
+        8,
+        3691,
+        3693,
+    ]
+    async with (
+        tractor.open_nursery() as an,
+        trio.open_nursery() as tn,
+    ):
+        for i, n in enumerate(inputs):
+            tn.start_soon(check, an, n, i)
+
+    for n, prime in sorted(results.items()):
+        print(f'{n} is prime: {prime}')
+
+
+if __name__ == '__main__':
+    trio.run(main)