feat(scheduler-core): wire builtin_fcfs as registered default picker (task 2.4)

Registers builtin_fcfs at scheduler startup and makes it the default
job_pick_plugin, replacing the hardcoded "xce_sched_fcfs" literal in
sched_job_picker() with a fail-loud lookup through sched_resolve_job_picker().
The picker thread now branches legacy job_elect_fn plugins (delegated
unchanged, preserving their self-contained wbuf-driven loop) from native
xce_job_picker_iface_t pickers (driven by a new per-session loop), and the
allocator now calls on_dispatch/on_pend for every picker as Req 1 AC 4
requires. Closes task 2.3's flagged picker/allocator concurrency race with a
mutex on fcfs_plugin_data_t, and adds cancel points to next_job().

src/scheduler/meson.build

@@ -7,6 +7,7 @@ scheduler_srcs = [
   'src/plugin.c',
   'src/resource.c',
   'src/plugins/fcfs.c',
+  'src/plugins/builtin_fcfs.c',
   'src/plugins/seqjob_allocator.c',
   'src/plugins/parjob_allocator.c',
   'src/plugins/limit.c',