feat(scheduler-core): pin jbuf capacity + record kvmgr determinism audit (task 4.2)

Hardcodes jbuf's cbuf capacity to the literal 1 in xce_sched_core_init()
(no parameter path, no local variable to drift), backed by a regression
test. Audits every kvmgr iteration reachable from the scheduling loop
(jobs/queues/users/projects): only jobs is ever iterated, via xzarray's
deterministic ascending-slot walk with no process-random seeding —
findings recorded durably as a comment above xce_sched_core_init().
core->workers iteration-order stability is flagged as an open follow-up
(scheduler-allocation's territory), not this task's scope.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019dw5WnN3DXStaEaPYfACU1

src/scheduler/testsuite/meson.build

@@ -203,6 +203,28 @@ sched_core_isolation_test = executable(
              + '"'],
     install: false)
 
+# Regression guard for task 4.2's jbuf-capacity half (scheduler-core spec,
+# Req 5 AC 7): "cbuf capacity=1 serialization ... this capacity is a
+# determinism invariant, not a tuning knob." Same white-box #include
+# pattern as test_picker_config_fields.c (pulls the real
+# xce_sched_core_init()/xce_sched_core_fini() from ../src/core.c into this
+# TU) PLUS the source-text-scan genre used by test_picker_core_loop_boundary_test/
+# test_core_isolation_test above -- a real runtime call proving
+# core.jbuf.ncap == 1, and a scan of core.c's actual on-disk text proving
+# the cbuf_init() capacity argument is the literal 1 (never a variable),
+# that the former `njobs` local is gone, and that the separate/unrelated
+# `ndecisions` FIXME was left untouched. XCE_CORE_C_PATH reuses the same
+# absolute-path constant as sched_core_isolation_test above.
+sched_core_jbuf_capacity_pin_test = executable(
+    'sched_core_jbuf_capacity_pin_test',
+    ['test_core_jbuf_capacity_pin.c'],
+    include_directories:[incdir],
+    dependencies: [scheduler_test_deps],
+    c_args: ['-DXCE_CORE_C_PATH="'
+             + (meson.current_source_dir() / '..' / 'src' / 'core.c')
+             + '"'],
+    install: false)
+
 # Register as meson tests so `meson test --suite scheduler` runs them all.
 # detect_leaks=0 matches the suite convention (fixtures are not torn down).
 test('scheduler test', scheduler_test, suite: 'scheduler',
@@ -236,3 +258,6 @@ test('scheduler pendreason factory audit test',
 test('scheduler core isolation test',
      sched_core_isolation_test, suite: 'scheduler',
      env: ['ASAN_OPTIONS=detect_leaks=0'])
+test('scheduler core jbuf capacity pin test',
+     sched_core_jbuf_capacity_pin_test, suite: 'scheduler',
+     env: ['ASAN_OPTIONS=detect_leaks=0'])