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
plugin_register() now adapts an old-style job_elect_fn-only plugin to
xce_job_picker_iface_t via legacy_picker_wrapper_iface (next_job()
forwards to job_elect_fn; other hooks are safe no-ops covered elsewhere
in the existing lifecycle), logging a one-time deprecation warning. A
plugin that already sets picker_iface directly skips the wrapper and
the warning. Reuses the existing plugin_register()/key_find() mechanism
-- no parallel registration system.
Requirements: 1.6, 2.5
