Make sub-spawn strategy toggleable in `we_are_processes`

The #470 boot-latency example hard-coded spawning each `worker_<i>`
subactor concurrently from a bg `trio.Task` (so each child's cold
`import tractor` overlaps). Add a `main()` `spawn_subs_in_bg_tasks`
flag so the serial-spawn path can be demo'd/compared too: flip it
`False` to `start_actor()` each sub inline in the loop before
handing the ready `Portal` to the bg task.

Deats,
- factor an `open_ep(ptl, i)` helper out of `spawn_and_open_ep()` -
  just the `Portal.open_context()` + `wait_for_result()` half, now
  that the spawn step is caller-optional.
- `spawn_and_open_ep()` grows a `maybe_ptl: Portal|None = None`
  param: spawn the subactor itself when unset (bg-task path), OW
  reuse the pre-spawned one (serial path).
- move the "overlap cold imports" rationale comment onto the new
  `main()` param where the toggle now lives.

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

examples/parallelism/we_are_processes.py

@@ -23,18 +23,10 @@ async def endpoint(
     await trio.sleep_forever()
 
 
-async def spawn_and_open_ep(
-    an: tractor.ActorNursery,
+async def open_ep(
+    ptl: tractor.Portal,
     i: int,
 ) -> None:
-    '''
-    Spawn a subactor, start a remote `endpoint()`-task in it.
-
-    '''
-    ptl: tractor.Portal = await an.start_actor(
-        name=f'worker_{i}',
-        enable_modules=[__name__],
-    )
     ctx: tractor.Context
     async with ptl.open_context(endpoint) as (
         ctx,
@@ -47,7 +39,33 @@ async def spawn_and_open_ep(
         await ctx.wait_for_result()
 
 
-async def main():
+async def spawn_and_open_ep(
+    an: tractor.ActorNursery,
+    i: int,
+    maybe_ptl: tractor.Portal|None = None,
+) -> None:
+    '''
+    Spawn a subactor, start a remote `endpoint()`-task in it.
+
+    '''
+    if maybe_ptl is None:
+        maybe_ptl: tractor.Portal = await an.start_actor(
+            name=f'worker_{i}',
+            enable_modules=[__name__],
+        )
+    await open_ep(
+        ptl=maybe_ptl,
+        i=i,
+    )
+
+
+async def main(
+    # spawn subs concurrently (in bg `trio.Task`s) so each
+    # actor's cold `import tractor` (~0.4s, see #470) overlaps
+    # instead of stacking; once forkserver (#463) lands, spawn
+    # is cheap enough to just loop sequentially.
+    spawn_subs_in_bg_tasks: bool = True,
+):
     '''
     Spawn a subactor-per-CPU then self-destruct the cluster.
 
@@ -60,17 +78,21 @@ async def main():
             # https://github.com/goodboy/tractor/pull/463
             # start_method='main_thread_forkserver',
         ) as an,
-        # spawn subs concurrently (in bg `trio.Task`s) so each
-        # actor's cold `import tractor` (~0.4s, see #470) overlaps
-        # instead of stacking; once forkserver (#463) lands, spawn
-        # is cheap enough to just loop sequentially.
         trio.open_nursery() as tn,
     ):
         for i in range(cpu_count()):
+
+            maybe_ptl: tractor.Portal|None = None
+            if not spawn_subs_in_bg_tasks:
+                maybe_ptl: tractor.Portal = await an.start_actor(
+                    name=f'worker_{i}',
+                    enable_modules=[__name__],
+                )
             tn.start_soon(
                 spawn_and_open_ep,
                 an,
                 i,
+                maybe_ptl,
             )
         destruct_in: int = 2
         print(