feat(scheduler-core): build normal-mode driver event translation path (task 5.1)

Adds driver.h/driver.c: xce_sched_driver_translate_pending_events()
drains core->pend_updates via the existing dataq-swap pattern (same
mechanism sched_update() uses), translates all 10 xce_sched_update_
request_t op types into the matching xce_core_event_t, and pushes the
batch through xce_core_push_events(). Scoped to xce_sched_core_t*
alone -- pend_updates already lives on core since task 4.1, and this
task is translation-only (no decision publishing, no mode-selection,
no recording -- all later work). handler.c (the enqueue side) is
untouched. First task in Group 5 (Core: Normal-Mode Driver).

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

src/scheduler/meson.build

@@ -6,6 +6,7 @@ scheduler_srcs = [
   'src/core_events.c',
   'src/core_session.c',
   'src/core_clock.c',
+  'src/driver.c',
   'src/decision.c',
   'src/schedobj.c',
   'src/plugin.c',