Use `.wait_for_result()` in dated examples

Swap the deprecated `portal.result()` calls for the modern
`.wait_for_result()` spelling in,
- `a_trynamic_first_scene.py` (x2)
- `actor_spawning_and_causality.py`
- `parallelism/single_func.py`

These 3 are literalinclude'd by the new docs tree so the rendered
code must teach the current api; the `debugging/` set still calls
`.result()` (pexpect pattern-matched tests, left for a follow-up
sweep).

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

examples/a_trynamic_first_scene.py

@@ -35,8 +35,8 @@ async def main():
             name='gretchen',
             other_actor='donny',
         )
-        print(await gretchen.result())
+        print(await gretchen.wait_for_result())
-        print(await donny.result())
+        print(await donny.wait_for_result())
         print("CUTTTT CUUTT CUT!!! Donny!! You're supposed to say...")
 
 

examples/actor_spawning_and_causality.py

@@ -20,7 +20,7 @@ async def main():
     # The ``async with`` will unblock here since the 'some_linguist'
     # actor has completed its main task ``cellar_door``.
 
-    print(await portal.result())
+    print(await portal.wait_for_result())
 
 
 if __name__ == '__main__':

examples/parallelism/single_func.py

@@ -33,7 +33,7 @@ async def main():
         await burn_cpu()
 
         # wait on result from target function
-        pid = await portal.result()
+        pid = await portal.wait_for_result()
 
     # end of nursery block
     print(f"Collected subproc {pid}")