feat(scheduler-core): implement core event batch push interface (task 4.3)

Defines xce_core_event_type_t/xce_core_event_t (9 event kinds: config,
job, worker, resource, resval full/delta, job state change, job update,
queue update, job rusage) in core.h. Implements xce_core_push_events()
in a new core_events.c (split out of core.c to stay under the 800-line
cap): rejects -EBUSY mid-session, otherwise applies each event's core-
state mutation and fires the corresponding picker hook synchronously,
exactly once per event, before returning. CONFIG_PUSH decomposes the
pushed config's embedded queues/resources/resvals via shared helpers
(also reused by QUEUE_UPDATE's upsert path), mirroring the existing
sched_init_config()/sched_init_resources()/sched_init_sharedInst()/
sched_init_queues(). Narrows sched_resolve_job_picker() to take
xce_sched_core_t* (pure type change, zero behavior change). Purely
additive — not wired into the live scheduling path yet (task 5.x).

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

src/scheduler/testsuite/meson.build

@@ -225,6 +225,22 @@ sched_core_jbuf_capacity_pin_test = executable(
              + '"'],
     install: false)
 
+# Contract test for the core event interface and batch event push
+# (scheduler-core spec, task 4.3, Req 5 AC 4 / AC 8): xce_core_push_events()
+# fires the resolved picker's corresponding state-change hook exactly once
+# per event, synchronously, before it returns, and rejects with -EBUSY (no
+# event applied) when a session is active. Same white-box #include pattern
+# as test_core_jbuf_capacity_pin.c/test_core_isolation.c (pulls the real
+# ../src/core.c into this TU) -- see the test's own header comment for why
+# it provides its own real sched_resolve_job_picker() rather than
+# #include-ing schedobj.c.
+sched_core_events_test = executable(
+    'sched_core_events_test',
+    ['test_core_events.c'],
+    include_directories:[incdir],
+    dependencies: [scheduler_test_deps],
+    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',
@@ -261,3 +277,6 @@ test('scheduler core isolation test',
 test('scheduler core jbuf capacity pin test',
      sched_core_jbuf_capacity_pin_test, suite: 'scheduler',
      env: ['ASAN_OPTIONS=detect_leaks=0'])
+test('scheduler core events test',
+     sched_core_events_test, suite: 'scheduler',
+     env: ['ASAN_OPTIONS=detect_leaks=0'])