Fix test_capi and test_cmd_line regressions on parallel-gc-dev

Two parallel-GC-related test failures, both PR-blockers for upstream:

1. test_capi: test_barrier_capacity_zero auto-discovery aborts the test_capi
   runner. _PyGCBarrier_Init asserts capacity > 0 and aborts the process;
   the test helper exists to trigger that assertion in a SUBPROCESS via
   test_gc_ws_deque.py. test_capi's auto-discovery of test_* functions in
   _testinternalcapi called it in-process and crashed the runner.

   Fix: rename test_barrier_capacity_zero -> unsafe_barrier_capacity_zero
   in Modules/_testinternalcapi/test_ws_deque.c and the corresponding
   subprocess invocation in Lib/test/test_gc_ws_deque.py.

   This is the 3.15 equivalent of commit d049fc8 on parallel_gc_backport
   (which fixed the same bug in the 3.12-backport tree). Cherry-pick was
   not possible because file structure differs: 3.12 has a single
   _testinternalcapi.c, 3.15 has Modules/_testinternalcapi/test_ws_deque.c.
   Applied as equivalent manual edit.

2. test_cmd_line: test_help_env and test_help_xoptions fail because the
   parallel GC additions to Python/initconfig.c (PYTHONPARALLELGC env var
   and -X parallel_gc option) were inserted at the wrong alphabetical
   position. The tests verify that env vars and X options appear in
   sort-order (underscore-stripped, lowercased).

   Fix: move PYTHONPARALLELGC to between PYTHONOPTIMIZE and
   PYTHON_PERF_JIT_SUPPORT. Move -X parallel_gc to between -X
   no_debug_ranges and -X pathconfig_warnings.

Verification:
- test_capi: 1356 tests pass (was: aborted)
- test_cmd_line: 76 tests pass (was: 2 failed)
- Full GIL test suite: 481 OK, 0 failures (was: 477 OK, 2 failed)

Part of 29-05-2026 3.15 consolidation. See:
  ~/claude_docs/29-05-2026-parallel-gc-3.15-consolidation-plan.md
  ~/claude_docs/parallel_gc_pr/29-05-2026-parallel-gc-3.15-consolidation-progress.md

Author: SonicField

Lib/test/test_gc_ws_deque.py

@@ -102,7 +102,7 @@ def test_capacity_zero(self):
         Falsifiability: removing assert(capacity > 0) from _PyGCBarrier_Init
         causes this test to hang (Wait decrements num_left=0 to UINT_MAX).
         """
-        code = "import _testinternalcapi; _testinternalcapi.test_barrier_capacity_zero()"
+        code = "import _testinternalcapi; _testinternalcapi.unsafe_barrier_capacity_zero()"
         result = subprocess.run(
             [sys.executable, "-c", code],
             capture_output=True, timeout=5

Modules/_testinternalcapi/test_ws_deque.c

@@ -526,8 +526,12 @@ test_ws_deque_concurrent_push_steal(PyObject *self, PyObject *Py_UNUSED(ignored)
 // ============================================================================
 
 // T3-F1: Init with capacity=0 should trigger assertion
+//
+// NOTE: Named with unsafe_ prefix to prevent test_capi auto-discovery.
+// This function triggers assert(capacity > 0) and aborts the process.
+// Only called from test_gc_ws_deque.py via subprocess.
 static PyObject *
-test_barrier_capacity_zero(PyObject *self, PyObject *Py_UNUSED(ignored))
+unsafe_barrier_capacity_zero(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
     _PyGCBarrier barrier;
     // This should trigger assert(capacity > 0) and abort
@@ -1328,7 +1332,7 @@ static PyMethodDef test_methods[] = {
     {"test_ws_deque_concurrent_push_steal", test_ws_deque_concurrent_push_steal, METH_NOARGS, NULL},
 
     // Barrier tests (T3-F1, T3-F9)
-    {"test_barrier_capacity_zero", test_barrier_capacity_zero, METH_NOARGS, NULL},
+    {"unsafe_barrier_capacity_zero", unsafe_barrier_capacity_zero, METH_NOARGS, NULL},
     {"test_barrier_basic", test_barrier_basic, METH_NOARGS, NULL},
     {"test_barrier_multiple_rounds", test_barrier_multiple_rounds, METH_NOARGS, NULL},
     {"test_barrier_epoch_distinguishes", test_barrier_epoch_distinguishes, METH_NOARGS, NULL},

Python/initconfig.c

@@ -325,10 +325,6 @@ The following implementation-specific options are available:\n\
 #ifdef Py_GIL_DISABLED
 "-X gil=[0|1]: enable (1) or disable (0) the GIL; also PYTHON_GIL\n"
 #endif
-#if defined(Py_PARALLEL_GC) || defined(Py_GIL_DISABLED)
-"-X parallel_gc=N: enable parallel GC with N workers;\n\
-         also PYTHONPARALLELGC\n"
-#endif
 "\
 -X importtime[=2]: show how long each import takes; use -X importtime=2 to\n\
          log imports of already-loaded modules; also PYTHONPROFILEIMPORTTIME\n\
@@ -338,6 +334,12 @@ The following implementation-specific options are available:\n\
          default is normal; also PYTHON_LAZY_IMPORTS\n\
 -X no_debug_ranges: don't include extra location information in code objects;\n\
          also PYTHONNODEBUGRANGES\n\
+"
+#if defined(Py_PARALLEL_GC) || defined(Py_GIL_DISABLED)
+"-X parallel_gc=N: enable parallel GC with N workers;\n\
+         also PYTHONPARALLELGC\n"
+#endif
+"\
 -X pathconfig_warnings=[0|1]: if true (1) then path configuration is allowed\n\
          to log warnings into stderr; if false (0) suppress these warnings;\n\
          set to true by default; also PYTHON_PATHCONFIG_WARNINGS\n\
@@ -434,9 +436,6 @@ static const char usage_envvars[] =
 #ifdef Py_GIL_DISABLED
 "PYTHON_GIL      : when set to 0, disables the GIL (-X gil)\n"
 #endif
-#if defined(Py_PARALLEL_GC) || defined(Py_GIL_DISABLED)
-"PYTHONPARALLELGC: enable parallel GC with N workers (-X parallel_gc=N)\n"
-#endif
 "PYTHONINSPECT   : inspect interactively after running script (-i)\n"
 "PYTHONINTMAXSTRDIGITS: limit the size of int<->str conversions;\n"
 "                  0 disables the limit (-X int_max_str_digits=N)\n"
@@ -445,6 +444,9 @@ static const char usage_envvars[] =
 "                  (-X no_debug_ranges)\n"
 "PYTHONNOUSERSITE: disable user site directory (-s)\n"
 "PYTHONOPTIMIZE  : enable level 1 optimizations (-O)\n"
+#if defined(Py_PARALLEL_GC) || defined(Py_GIL_DISABLED)
+"PYTHONPARALLELGC: enable parallel GC with N workers (-X parallel_gc=N)\n"
+#endif
 "PYTHON_PERF_JIT_SUPPORT: enable Linux \"perf\" profiler support with JIT\n"
 "                  (-X perf_jit)\n"
 "PYTHONPERFSUPPORT: support the Linux \"perf\" profiler (-X perf)\n"