fix: serialize scheduler transitions and coalesce misfires

tests/scheduling/test_poll.py

@@ -173,6 +173,82 @@ def test_latest_coalesces_to_one_candidate_and_no_double_admit() -> None:
         sched.ownership.release()
 
 
+def test_latest_short_downtime_does_not_burst_admit(tmp_path: Path) -> None:
+    """Latest misfire coalesces several bounded missed starts to one run."""
+    sched, store, runs, sources = _harness(tmp_path, script={"*": "complete"})
+    start = ts(2026, 9, 8, 9, 0)
+    now = ts(2026, 9, 8, 12, 0)
+    _add(
+        sched,
+        store,
+        sources,
+        "m",
+        PeriodicSource(timedelta(hours=1), start),
+        start,
+        misfire="latest",
+    )
+
+    sched.poll(now)
+
+    admitted = [row for row in _history(store, "m") if row["kind"] == "admitted"]
+    assert len(admitted) == 1
+    assert datetime.fromisoformat(admitted[0]["resolved_at"]) == now
+    sched.ownership.release()
+
+
+def test_schedule_edit_between_poll_snapshot_and_admission_cannot_overwrite_terms(
+    tmp_path: Path,
+) -> None:
+    """An edit between the poll snapshot and admission wins authoritatively."""
+
+    class EditOnPollRead(FileScheduleStore):
+        def __init__(self, root: Path) -> None:
+            super().__init__(root)
+            self.armed = False
+
+        def get_schedule(self, schedule_id: str) -> Schedule:
+            current = super().get_schedule(schedule_id)
+            if self.armed and schedule_id == "a":
+                self.armed = False
+                edited = current.model_copy(
+                    update={
+                        "revision": current.revision + 1,
+                        "updated_at": current.updated_at + timedelta(seconds=1),
+                    }
+                )
+                super().update_schedule(edited, expected_revision=current.revision)
+                return current
+            return current
+
+    sched_store = EditOnPollRead(tmp_path / "sched")
+    run_store = FileRunStore(tmp_path / "runs")
+    sources: dict[str, Any] = {}
+    delegate = SchedulePreparer(
+        DictDeployments({"dep-1": {"rev": 1, "required": []}}),
+        fixture_environment,
+    )
+
+    sched = Scheduler(
+        schedule_store=sched_store,
+        run_store=run_store,
+        sources=sources,
+        capacity=1,
+        preparer=delegate,
+        dispatcher=ScriptedDispatcher({"*": "complete"}),
+        ownership=SchedulerOwnership(tmp_path, owner="test").acquire(),
+    )
+    t0 = ts(2026, 9, 8, 12, 0)
+    _add(sched, sched_store, sources, "a", OneShotSource(t0), t0 - timedelta(hours=1))
+    sched_store.armed = True
+    try:
+        result = sched.poll(t0)
+        assert result["a"] == "admit:schedule-changed"
+        assert sched_store.get_schedule("a").revision == 2
+        assert run_store.list_admissions() == []
+    finally:
+        sched.ownership.release()
+
+
 def test_parallel_limits_and_interrupted_slots() -> None:
     import tempfile