feat(scheduler-core): wire decision publishing and mode-selection into main.c (task 5.2)

Rewires main.c's live scheduling loop onto the new core-driven
architecture: xce_sched_driver_translate_pending_events() (task 5.1)
-> xce_core_begin_session() -> plugin_begin_session()+sched_account_jobs()
-> xce_core_signal_go() -> decision-drain via the registered
xce_core_decision_cb_t -> plugin_end_session() -> xce_core_end_session(),
all in the new exported xce_sched_driver_run_normal_mode_round()
(driver.c), replacing the old inline sched_update()+sched_begin_session()
+sched_go()+switch+sched_end_session() sequence.

xce_core_go() (task 4.4) is split into xce_core_begin_session()/
xce_core_signal_go() -- a pure decomposition (xce_core_go() itself is
now defined in terms of both, task 4.4's approved test suite passes
unmodified) -- specifically so the driver can interleave
plugin_begin_session()/sched_account_jobs() in the exact safety-critical
window task 4.4 deliberately excluded from pure-core scope: after the
resval reset, strictly before the picker thread is signaled. This is
empirically proven by test_driver_session_ordering.c via real
cross-thread happens-before assertions, not just documented.

xce_sched_driver_on_decision() is the new registered callback:
DISPATCH/PEND forward to the existing sched_publish_dispatch()/
sched_publish_pend_reason(); PREEMPT fails loudly (-ENOSYS, logged)
since no xcecli_preempt() client API exists and nothing in the live
allocator produces one yet (scheduler-preemption's territory).

Adds a --sim CLI flag (parse-only, loud not-yet-implemented failure,
since SimDriver is Phase 3).

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

src/scheduler/testsuite/test_driver_translate.c

@@ -67,6 +67,75 @@ void kill_sched_project(void* arg) { (void)arg; }
 void kill_sched_resval(xce_val_t* val) { (void)val; }
 void plugin_unregister(void* arg) { (void)arg; }
 
+/* Link-satisfying stubs for scheduler.c's sched_publish_dispatch()/
+ * sched_publish_pend_reason() -- as of scheduler-core spec task 5.2,
+ * driver.c's (real, #included above) xce_sched_driver_on_decision() calls
+ * both, but this file never exercises that function (it is out of THIS
+ * file's scope -- see test_driver_decision_cb.c) and never links
+ * scheduler.c (same rationale as the sched_resolve_job_picker
+ * reimplementation below: this file's own fixtures never need a real
+ * xcecli_ctx_t/apictx). Never actually invoked at runtime by any test in
+ * this file. */
+int sched_publish_dispatch(xce_scheduler_t* sched, xce_job_dispatch_t* d)
+{
+    (void)sched;
+    (void)d;
+    return 0;
+}
+
+int sched_publish_pend_reason(xce_scheduler_t* sched, xce_preason_set_t* r)
+{
+    (void)sched;
+    (void)r;
+    return 0;
+}
+
+/* Link-satisfying stubs: as of scheduler-core spec task 5.2,
+ * xce_sched_driver_run_normal_mode_round() (driver.c, real, #included
+ * above, alongside xce_sched_driver_translate_pending_events() this file
+ * DOES exercise) references xce_core_begin_session()/xce_core_signal_go()/
+ * xce_core_end_session() (core_session.c) and plugin_begin_session()/
+ * plugin_end_session() (plugin.c) / sched_account_jobs() (scheduler.c).
+ * This file never exercises xce_sched_driver_run_normal_mode_round() itself
+ * (out of THIS file's scope -- see test_driver_session_ordering.c) and
+ * never links core_session.c/plugin.c/scheduler.c. Never actually invoked
+ * at runtime by any test in this file. */
+int xce_core_begin_session(xce_sched_core_t* core)
+{
+    (void)core;
+    return 0;
+}
+
+int xce_core_signal_go(xce_sched_core_t* core)
+{
+    (void)core;
+    return 0;
+}
+
+int xce_core_end_session(xce_sched_core_t* core)
+{
+    (void)core;
+    return 0;
+}
+
+int plugin_begin_session(void* arg)
+{
+    (void)arg;
+    return 0;
+}
+
+int plugin_end_session(void* arg)
+{
+    (void)arg;
+    return 0;
+}
+
+int sched_account_jobs(xce_scheduler_t* sched)
+{
+    (void)sched;
+    return 0;
+}
+
 /* Real reimplementation of schedobj.c's sched_resolve_job_picker -- see
  * file header comment (same rationale as test_core_events.c's own copy). */
 int sched_resolve_job_picker(xce_sched_core_t* core, xce_sched_plugin_t** out)