feat(scheduler-sim-live-allocation): extract shared picker/allocator into sched_election.c (task 3.1)

Relocates sched_job_picker()/sched_allocator() out of scheduler.c
(1382 lines, already over this project's 800-line cap) into a new
sched_election.c, retyped to xce_sched_core_t* -- the single
implementation both drivers will spawn (task 4.1 wires simulation
mode onto it). Independently verified bit-for-bit identical to the
pre-relocation bodies modulo the sched->/core-> rename; no statement
reordered, no branch changed.

sched_make_allocation() becomes extern (was static, sole caller moved
translation units); the old commit_decision() thin wrapper is deleted,
its sole caller now calls sched_commit_decision() directly with
identical semantics. sched_init() updated to spawn the relocated pair
against &(sched->core).

Adds an end-to-end fixture test proving a job flows from picked to a
committed decision against bare (non-xce_scheduler_t) core state.
Routes around, rather than fixes, two pre-existing out-of-boundary
gaps this fixture exposed (seqjob/parjob allocator init still reading
plugin->sched; make_sched_job()'s parse path not threading
maxtasks/mintasks) -- both documented in the test's own comments for
future follow-up.

sched_election.c added to the normal build only; sched_sim_isolation_lib
membership deferred to task 4.1, which is where sim mode actually
starts spawning these threads.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLSNJhNoULvVcUhXEFafQC

src/scheduler/meson.build

@@ -2,6 +2,7 @@ scheduler_srcs = [
   'src/main.c',
   'src/handler.c',
   'src/scheduler.c',
+  'src/sched_election.c',
   'src/core.c',
   'src/core_events.c',
   'src/core_session.c',