Hoist proc-title prefix to `_def_prefix` const

Make the sub-actor proc-title prefix a single
authoritative constant (`_proctitle._def_prefix`) so
the reap-recognition markers and `xontrib` banner pick
it up automatically — one place to flip the prefix
shape going fwd.

Deats,
- `_proctitle._def_prefix: str = '_subactor'`. New
  module-level const consumed by everything that needs
  to know the prefix.
- `set_actor_proctitle(actor, prefix=_def_prefix)`:
  takes an explicit `prefix` arg (default = the const)
  so callers can override per-spawn if they want.
- Default proc-title format:
  `'tractor[<reprol>]'` → `f'{prefix}[<reprol>]'`
  i.e. `_subactor[<reprol>]` by default.
- `_testing/_reap.py`: cmdline + comm markers source
  the prefix from `_proctitle._def_prefix` instead of
  the hardcoded `'tractor['`. So
  `_is_tractor_subactor()` tracks the const
  automatically.
- `xontrib/tractor_diag.xsh`: `acli.reap` orphan-mode
  banner now interpolates the
  `_TRACTOR_PROC_CMDLINE_MARKERS` tuple directly so
  the human-readable mode line stays in sync if the
  prefix shape changes again.

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

(cherry picked from commit 3a45dbd503)

tests/devx/test_proctitle.py

@@ -8,9 +8,9 @@ after `Actor` construction, so any spawned sub-actor process
 should:
 
   - have `argv[0]` (== `/proc/<pid>/cmdline`) start with
-    `tractor[<aid.reprol()>]`
+    `<_def_prefix>[<aid.reprol()>]` (currently `_subactor[…]`)
-  - have `/proc/<pid>/comm` start with `tractor[` (kernel
+  - have `/proc/<pid>/comm` start with `<_def_prefix>[`
-    truncates to ~15 bytes)
+    (kernel truncates to ~15 bytes)
   - be detected as a tractor sub-actor by
     `_is_tractor_subactor(pid)` via the cmdline marker.
 
@@ -27,7 +27,10 @@ import trio
 import tractor
 
 from tractor.runtime._runtime import Actor
-from tractor.devx._proctitle import set_actor_proctitle
+from tractor.devx._proctitle import (
+    set_actor_proctitle,
+    _def_prefix,
+)
 from tractor._testing._reap import (
     _is_tractor_subactor,
     _read_cmdline,
@@ -41,8 +44,9 @@ _non_linux: bool = platform.system() != 'Linux'
 def test_set_actor_proctitle_format():
     '''
     `set_actor_proctitle()` returns the canonical
-    `tractor[<aid.reprol()>]` form and actually mutates
+    `<_def_prefix>[<aid.reprol()>]` form (currently
-    the running proc's title.
+    `_subactor[…]`) and actually mutates the running
+    proc's title.
 
     '''
     pytest.importorskip(
@@ -60,12 +64,14 @@ def test_set_actor_proctitle_format():
         )
         title: str = set_actor_proctitle(actor)
 
-        # canonical wrapping: `tractor[<aid.reprol()>]`. We
+        # canonical wrapping: `<_def_prefix>[<aid.reprol()>]`.
-        # compare against the runtime-computed `reprol()`
+        # We source BOTH the prefix (`_def_prefix`) and the
-        # rather than a hard-coded value so the test stays
+        # runtime-computed `reprol()` rather than hard-coding,
-        # decoupled from `Aid.reprol()`'s internal format
+        # so the test stays decoupled from the prefix shape
-        # (currently `<name>@<pid>`, but could evolve).
+        # (flipped to `_subactor` in `3a45dbd5`) AND from
-        expected: str = f'tractor[{actor.aid.reprol()}]'
+        # `Aid.reprol()`'s internal format (currently
+        # `<name>@<pid>`, but could evolve).
+        expected: str = f'{_def_prefix}[{actor.aid.reprol()}]'
         assert title == expected
         # sanity: the actor's name must be in the title
         # somewhere (so a future `reprol()` change that
@@ -140,15 +146,17 @@ def test_subactor_proctitle_visible_via_proc():
     )
 
     pid, info = matched[0]
-    # canonical proctitle prefix in cmdline (full form)
+    # canonical proctitle prefix in cmdline (full form);
-    assert info['cmdline'].startswith('tractor[proctitle_boi@'), (
+    # prefix sourced from `_def_prefix` so it tracks the
-        f'cmdline missing `tractor[proctitle_boi@…]` prefix: '
+    # `3a45dbd5` flip (`tractor[` -> `_subactor[`).
+    assert info['cmdline'].startswith(f'{_def_prefix}[proctitle_boi@'), (
+        f'cmdline missing `{_def_prefix}[proctitle_boi@…]` prefix: '
         f'{info["cmdline"]!r}'
     )
     # comm is kernel-truncated to ~15 bytes — just check the
-    # `tractor[` prefix made it.
+    # `<_def_prefix>[` prefix made it.
-    assert info['comm'].startswith('tractor['), (
+    assert info['comm'].startswith(f'{_def_prefix}['), (
-        f'comm missing `tractor[` prefix: {info["comm"]!r}'
+        f'comm missing `{_def_prefix}[` prefix: {info["comm"]!r}'
     )
     # intrinsic-signal detector should match.
     assert info['is_tractor'] is True

tractor/_testing/_reap.py

@@ -90,7 +90,6 @@ keys are caller-defined).
 
 '''
 from __future__ import annotations
-
 import os
 import pathlib
 import re
@@ -99,6 +98,9 @@ import stat
 import sys
 import time
 
+
+from tractor.devx import _proctitle
+
 # `/dev/shm` is the POSIX-shm filesystem on Linux + FreeBSD.
 # macOS uses `shm_open` syscalls without a fs-visible path,
 # so the shm helpers refuse to run there.
@@ -230,9 +232,9 @@ def _read_comm(pid: int) -> str:
 #   while `cmdline` for zombies often reads as empty.
 _TRACTOR_PROC_CMDLINE_MARKERS: tuple[str, ...] = (
     'tractor._child',
-    'tractor[',
+    _proctitle._def_prefix,
 )
-_TRACTOR_PROC_COMM_MARKER: str = 'tractor['
+_TRACTOR_PROC_COMM_MARKER: str = _proctitle._def_prefix
 
 
 def _is_tractor_subactor(pid: int) -> bool:

tractor/devx/_proctitle.py

@@ -24,7 +24,10 @@ which" at a glance without needing to read full
 `/proc/<pid>/cmdline`.
 
 Format:
-  ``tractor[<aid.reprol()>]``    e.g. ``tractor[doggy@1027301b]``
+  ``<_def_prefix>[<aid.reprol()>]``  e.g. ``_subactor[doggy@1027301b]``
+(prefix from the `_def_prefix` const, flipped `tractor` ->
+`_subactor` so sub-actor procs are visually distinct from the
+root in `ps`/`htop` and the reap-recognition markers.)
 
 Uses the canonical `Aid.reprol()` form
 (``<name>@<uuid_short>``) so the proc-title matches the
@@ -52,7 +55,13 @@ except ImportError:
     _stp = None
 
 
-def set_actor_proctitle(actor: 'Actor') -> str | None:
+_def_prefix: str = '_subactor'
+
+
+def set_actor_proctitle(
+    actor: 'Actor',
+    prefix: str = _def_prefix,
+) -> str | None:
     '''
     Set the calling process's proc-title to identify it as a
     tractor sub-actor.
@@ -69,6 +78,6 @@ def set_actor_proctitle(actor: 'Actor') -> str | None:
     if _stp is None:
         return None
 
-    title: str = f'tractor[{actor.aid.reprol()}]'
+    title: str = f'{prefix}[{actor.aid.reprol()}]'
     _stp.setproctitle(title)
     return title

xontrib/tractor_diag.xsh

@@ -488,6 +488,7 @@ def _tractor_reap(args):
         reap,
         reap_shm,
         reap_uds,
+        _TRACTOR_PROC_CMDLINE_MARKERS,
     )
 
     rc: int = 0
@@ -500,9 +501,8 @@ def _tractor_reap(args):
         else:
             pids = find_orphans()
             mode = (
-                'orphans (PPid==1, intrinsic '
+                f'orphans (PPid==1, intrinsic '
-                'cmdline/comm match — `tractor[…]` or '
+                f'cmdline/comm match — {_TRACTOR_PROC_CMDLINE_MARKERS}'
-                '`tractor._child`)'
             )
 
         if not pids: