Document `SharedMemory` × `subint_forkserver` incompat

New `ai/conc-anal/` doc: `mp.SharedMemory` is
fork-without-exec unsafe — child inherits parent's
`resource_tracker` fd → EBADF on first shm op;
leaked `/shm_list` cascades `FileExistsError`
across parametrize variants. Canonical CPython
issue class, NOT a tractor bug. Includes two
longer-term mitigation paths (reset inherited
tracker fd vs migrate off `mp.shared_memory`).

Also, update `tests/test_shm.py`:
- comment out `subint_forkserver` from skip list
- rewrite reason with precise failure-mode
  descriptions + link to the analysis doc

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

(cherry picked from commit c99d475d03)
(factored: dropped spawn-backend-only paths: ai/conc-anal/subint_forkserver_mp_shared_memory_issue.md)

tests/test_shm.py

@@ -16,10 +16,18 @@ from tractor.ipc._shm import (
 
 pytestmark = pytest.mark.skipon_spawn_backend(
     'subint',
+    # 'subint_forkserver',
     reason=(
-        'XXX SUBINT GIL-CONTENTION HANGING TEST XXX\n'
+        'subint: GIL-contention hanging class.\n'
-        'See oustanding issue(s)\n'
+        'subint_forkserver: `multiprocessing.SharedMemory` '
-        # TODO, put issue link!
+        'is fork-without-exec unsafe — child inherits parent\'s '
+        '`resource_tracker` fd → EBADF on first shm op '
+        '(`test_child_attaches_alot`); leaked `/shm_list` from '
+        'a "passing" run cascades into `FileExistsError` across '
+        'parametrize variants (`test_parent_writer_child_reader`). '
+        'Canonical CPython issue class, NOT a tractor bug; full '
+        'tracker doc:\n'
+        'ai/conc-anal/subint_forkserver_mp_shared_memory_issue.md'
     )
 )