feat(scheduler-core): extract xce_sched_core_t from monolithic scheduler struct (task 4.1)

Splits xce_scheduler_t along the controller-I/O seam: xce_sched_core_t
(new core.h/core.c) now holds all scheduling state (jobs/queues/workers/
users/projects/resource managers/plugins/three-thread-cbuf pipeline),
while xce_scheduler_t shrinks to {core; apictx; cluster_cfg}. Proven via
an nm-based link-time isolation test (zero netio/xcecli/idgen/delaymgr
symbols reachable from core.c) plus a reproducible wall-clock-call guard.

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

src/scheduler/testsuite/test_picker_pendreason_factory_audit.c

@@ -56,10 +56,12 @@
 #endif
 
 static const char* const SCANNED_RELATIVE_PATHS[] = {
+    "include/core.h",
     "include/decision.h",
     "include/plugin.h",
     "include/scheduler.h",
     "include/sched/types.h",
+    "src/core.c",
     "src/decision.c",
     "src/handler.c",
     "src/main.c",