fix(merge): restore bug-1490 job control + p2-hang flusher fix + B3 test reconciliation

Merge follow-up fixes found during comprehensive unit-test + live-cluster
verification of the scheduler-framework merge:

- apiserver/backend/clib.go (ControlJob): Rongsong's clib.go had reverted
  our bug-1490 fix — ctrl.action must be the capnp JobControl union
  DISCRIMINANT (0=siginfo,1=ckptoption), not the models.JOB_* constant
  (1/2/3/8), or every kill/stop/resume fails -EINVAL. Restored
  ctrl.action = C.int(jobCtrlDiscriminant(action)); verified live (job
  kill RUN->EXIT).

- controller/persist.c (fsync_worker): the merged bug-1563 no-progress
  backoff used the PRE-FLUSH `pending` snapshot, so a no-op flush (another
  path advanced durable_lsn to tip mid-flush) also slept wal_sync_ms and
  raced commit_engine_fini in teardown, hanging the p2_integration suite
  ~60% (bug-1515). Re-evaluate the level AFTER the flush (pending_after)
  so only a genuine sustained fdatasync failure backoffs. 20/20 clean p2
  runs (was 3/5 hang). Also carries the bug-1509 B3 RUN->UNKWN marking in
  persist_recover.

- scheduler/worker testsuite meson.build: add unity_dep parent includedir
  (unity.pc Cflags only add -I${includedir}/unity, so <unity/unity.h> was
  unresolvable) — same fix as src/lib/meson.build.

- test_handlers.c: re-point notify.data.dispatchfail after the first
  sched_handle_notify NULLs it on success (code=0 branch was -EINVAL).

- test_controller_connector.c: give the rejoin-success fixtures a valid
  worker_list pointer (my bug-1509 resend_job_status deref was NULL-
  crashing).

- test_wal_job_dispatch_fact.c / test_dispatch_commit.c: Rongsong's
  WAL_JOB_DISPATCH replay tests asserted JOB_STAT_RUN after persist_recover,
  conflicting with bug-1509 B3 (RUN->UNKWN on restart). Assert JOB_IS_RUN
  (base RUN recovered, incl. bug-1678 allocs) + UNKWN flag instead.

All suites green: controller 1145/0, scheduler 57/57, worker 15/15,
lib 303/0, apiserver CGO build ok.

src/frontend/apiserver/backend/clib.go

@@ -664,7 +664,13 @@ func SubmitJob(job *models.JobInfo, arraySize int) error {
 func ControlJob(jobid uint64, action int, content string) error {
 	var ctrl C.xce_jobctrl_t
 	ctrl.jobid = C.uint64_t(jobid)
-	ctrl.action = C.int(action)
+	// ctrl.action is the capnp JobControl union DISCRIMINANT (0=siginfo,
+	// 1=ckptoption), NOT the models.JOB_* constant. bug-1490: setting it to
+	// JOB_STOP/RESUME/KILL/CHKPNT (1/2/3/8) made the controller's job_ctrl()
+	// select the wrong union branch (or none) and reject every control with
+	// -EINVAL, so hold/release/cancel never reached the job. (Re-applied on
+	// the merge branch — Rongsong's clib.go had reverted it to action.)
+	ctrl.action = C.int(jobCtrlDiscriminant(action))
 
 	// For signal-based actions set signo; for checkpoint set ckpt options.
 	// xce_jobctrl_t.u is a C union — we use a static C helper to set the