Un-hide stdlib primes example, add `trio` shim

Rename `parallelism/_concurrent_futures_primes.py` ->
`concurrent_futures_primes.py` so the example-runner
(`test_docs_examples.py`) stops skipping it (leading `_` =
excluded) and CI finally exercises the `concurrent.futures`
baseline we compare against in the new parallelism guide.

Deats,
- keep the original executor code verbatim in a sync
  `check_primes()` fn for clean docs excerpting,
- add module docstring + zero-arg `async def main()` +
  `trio.run(main)` guard per the runner conventions.

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

examples/parallelism/concurrent_futures_primes.py

@@ -1,7 +1,19 @@
+'''
+The pure-stdlib `concurrent.futures.ProcessPoolExecutor`
+primes demo (from the std docs) verbatim; the baseline twin
+of `concurrent_actors_primes.py`.
+
+The `async def main()` + `trio.run()` shim at the bottom only
+exists so the docs-example test runner can exercise this
+script; the executor code itself is untouched stdlib fare.
+
+'''
 import time
 import concurrent.futures
 import math
 
+import trio
+
 PRIMES = [
     112272535095293,
     112582705942171,
@@ -26,7 +38,7 @@ def is_prime(n):
     return True
 
 
-def main():
+def check_primes():
     with concurrent.futures.ProcessPoolExecutor() as executor:
         start = time.time()
 
@@ -36,8 +48,14 @@ def main():
         print(f'processing took {time.time() - start} seconds')
 
 
+async def main() -> None:
+    # thin shim: the pool blocks this (sole) trio task
+    # which is just fine for a one-shot baseline script.
+    check_primes()
+
+
 if __name__ == '__main__':
 
     start = time.time()
-    main()
+    trio.run(main)
     print(f'script took {time.time() - start} seconds')