Label forkserver child as `subint_forkserver`

Follow-up to 72d1b901 (was prev commit adding `debug_mode` for
`subint_forkserver`): that commit wired the runtime-side
`subint_forkserver` SpawnSpec-recv gate in `Actor._from_parent`, but the
`subint_forkserver_proc` child-target was still passing
`spawn_method='trio'` to `_trio_main` — so `Actor.pformat()` / log lines
would report the subactor as plain `'trio'` instead of the actual
parent-side spawn mechanism. Flip the label to `'subint_forkserver'`.

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

tractor/spawn/_subint_forkserver.py

@@ -655,12 +655,16 @@ async def subint_forkserver_proc(
             loglevel=loglevel,
             parent_addr=parent_addr,
             infect_asyncio=infect_asyncio,
-            # NOTE, from the child-side runtime's POV it's
-            # a regular trio actor — it uses `_trio_main`,
-            # receives `SpawnSpec` over IPC, etc. The
-            # `subint_forkserver` name is a property of HOW
-            # the parent spawned, not of what the child is.
-            spawn_method='trio',
+            # The child's runtime is trio-native (uses
+            # `_trio_main` + receives `SpawnSpec` over IPC),
+            # but label it with the actual parent-side spawn
+            # mechanism so `Actor.pformat()` / log lines
+            # reflect reality. Downstream runtime gates that
+            # key on `_spawn_method` group `subint_forkserver`
+            # alongside `trio`/`subint` where the SpawnSpec
+            # IPC handshake is concerned — see
+            # `runtime._runtime.Actor._from_parent()`.
+            spawn_method='subint_forkserver',
         )
         return 0