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/meson.build

@@ -175,6 +175,34 @@ sched_pendreason_factory_audit_test = executable(
              + '"'],
     install: false)
 
+# Regression guard for task 4.1's Observable (scheduler-core spec, Req
+# 5.1/5.2): "the core structure compiles and links with zero symbols from
+# the networking/API libraries ... a build-time or link-time check
+# confirms this." Same source-text-scan genre as
+# sched_picker_core_loop_boundary_test / sched_pendreason_factory_audit_test
+# above, PLUS a genuine link-time proof: it inspects the undefined-symbol
+# table (via nm(1)) of sched_core_isolation_lib (defined in the parent
+# meson.build -- core.c compiled alone, isolated from the rest of the
+# scheduler). link_with establishes the build-order dependency so that
+# archive exists before this test runs; XCE_CORE_LIB_PATH bakes in its
+# absolute path the same way XCE_SCHEDULER_C_PATH does above.
+sched_core_isolation_test = executable(
+    'sched_core_isolation_test',
+    ['test_core_isolation.c'],
+    include_directories:[incdir],
+    dependencies: [scheduler_test_deps],
+    link_with: [sched_core_isolation_lib],
+    c_args: ['-DXCE_CORE_H_PATH="'
+             + (meson.current_source_dir() / '..' / 'include' / 'core.h')
+             + '"',
+             '-DXCE_CORE_C_PATH="'
+             + (meson.current_source_dir() / '..' / 'src' / 'core.c')
+             + '"',
+             '-DXCE_CORE_LIB_PATH="'
+             + sched_core_isolation_lib.full_path()
+             + '"'],
+    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',
@@ -205,3 +233,6 @@ test('scheduler picker core loop boundary test',
 test('scheduler pendreason factory audit test',
      sched_pendreason_factory_audit_test, suite: 'scheduler',
      env: ['ASAN_OPTIONS=detect_leaks=0'])
+test('scheduler core isolation test',
+     sched_core_isolation_test, suite: 'scheduler',
+     env: ['ASAN_OPTIONS=detect_leaks=0'])