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().
Adds begin_session/next_job/end_session/on_dispatch/on_pend, driving a
dual-cursor walk (queue then job) over builtin_fcfs's private ordq_t
state. next_job() returns candidates in (queue priority desc, job
priority desc, queue_time asc) order and NULL once exhausted;
on_dispatch removes permanently, on_pend is a true no-op.
Review flagged an unsynchronized cross-thread access risk between
next_job() (picker thread) and on_dispatch/on_pend (allocator thread)
once wired live in task 2.4 — recorded in tasks.md Implementation Notes
for that task to close before registering builtin_fcfs as the default.
Requirements: 3.2, 3.4, 3.6
Adds on_queue_update, on_job_push, on_job_state_change, and on_job_update
to builtin_fcfs, driving the picker's private ordq_t candidate sets
directly (no core/sched queries), matching fcfs.c's existing ordering
semantics on the new finer-grained hook interface.
Requirements: 3.2
New src/scheduler/src/plugins/builtin_fcfs.c implements
builtin_fcfs_init()/fini() and the private fcfs_plugin_data_t (ordq_t
queues, per-queue ordq_t pendJobs) that all later builtin_fcfs hooks
build on. fcfs.c (today's live job_elect_fn-based picker) is untouched
-- still production, until task 2.4 flips the registration.
picker == xce_sched_plugin_t* calling convention (private state stored
in plugin->data) confirmed consistent with task 1.2's legacy wrapper.
Requirements: 3.1 (context), 3.3
