Reset post-fork `_state` in forkserver child

`os.fork()` inherits the parent's entire memory image,
including `tractor.runtime._state` globals that encode
"this process is the root actor" — `_runtime_vars`'s
`_is_root=True`, pre-populated `_root_mailbox` +
`_registry_addrs`, and the parent's `_current_actor`
singleton.

A fresh `exec`-based child starts with those globals at
their module-level defaults (all falsey/empty). The
forkserver child needs to match that shape BEFORE calling
`_actor_child_main()`, otherwise `Actor.__init__()` takes
the `is_root_process() == True` branch and pre-populates
`self.enable_modules`, which then trips
`assert not self.enable_modules` at the top of
`Actor._from_parent()` on the subsequent parent→child
`SpawnSpec` handshake.

Fix: at the start of `_child_target`, null
`_state._current_actor` and overwrite `_runtime_vars` with
a cold-root blank (`_is_root=False`, empty mailbox/addrs,
`_debug_mode=False`) before `_actor_child_main()` runs.

Found-via: `test_subint_forkserver_spawn_basic` hitting
the `enable_modules` assert on child-side runtime boot.

(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

@@ -548,6 +548,32 @@ async def subint_forkserver_proc(
         # every spawn — it's module-level in `_child` but
         # cheap enough to re-resolve here.
         from tractor._child import _actor_child_main
+        # XXX, fork inherits the parent's entire memory
+        # image — including `tractor.runtime._state` globals
+        # that encode "this process is the root actor":
+        #
+        # - `_runtime_vars['_is_root']` → True in parent
+        # - pre-populated `_root_mailbox`, `_registry_addrs`
+        # - the parent's `_current_actor` singleton
+        #
+        # A fresh `exec`-based child would start with the
+        # `_state` module's defaults (all falsey / empty).
+        # Replicate that here so the new child-side `Actor`
+        # sees a "cold" runtime — otherwise `Actor.__init__`
+        # takes the `is_root_process() == True` branch and
+        # pre-populates `self.enable_modules`, which then
+        # trips the `assert not self.enable_modules` gate at
+        # the top of `Actor._from_parent()` on the subsequent
+        # parent→child `SpawnSpec` handshake.
+        from tractor.runtime import _state
+        _state._current_actor = None
+        _state._runtime_vars.update({
+            '_is_root': False,
+            '_root_mailbox': (None, None),
+            '_root_addrs': [],
+            '_registry_addrs': [],
+            '_debug_mode': False,
+        })
         _actor_child_main(
             uid=uid,
             loglevel=loglevel,