feat(scheduler-core): implement session-start signal + decision callback (task 4.4)

Adds xce_core_go()/xce_core_end_session() (new core_session.c):
xce_core_go() is non-blocking, performs begin-session state reset
(select-eval-ctx, resval resets), signals the picker thread via wbuf,
and returns -EINVAL on a reentrant call while a session is already
active (proven a true no-op, not just an early return). Draining
core->decisions and invoking the driver's on_decision callback is the
driver's own responsibility between core_go() and core_end_session()
(confirmed driver-owned, not core-owned, per the Driver <-> Core
protocol). xce_core_set_decision_cb() registers the callback + driver
context as a separate setter, keeping xce_sched_core_init()'s live
sched_init() call site untouched. Adds XCE_DECISION_PREEMPT to
decision.h (jobid/victims[]/reason) alongside the existing DISPATCH/
PEND types. Additive only — sched_go/sched_begin_session/
sched_end_session/sched_allocator/main.c remain untouched.

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

@@ -62,6 +62,7 @@ static const char* const SCANNED_RELATIVE_PATHS[] = {
     "include/scheduler.h",
     "include/sched/types.h",
     "src/core.c",
+    "src/core_session.c",
     "src/decision.c",
     "src/handler.c",
     "src/main.c",