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