Wait for a deadline instead of polling for one

The timer thread promoted due work on a fixed one-second tick, so every
delayed item was 0-1000ms late whatever the load — measured on the house
at 705ms mean on a rollershutter stop, which is 2-4% of a 26-second
travel and accumulates in the position the motor node believes it is at.
It now sleeps to the soonest deadline and is woken when a nearer one is
scheduled, which measures 0.9ms end to end through Redis.

A promoted timer also went to the back of the queue. It goes into a due
lane of its own that `claim` reads first, so work that has waited out a
deadline is not held up by work that is merely queued.

Beside it, in the same code: seeding a message now bumps its version, so
a re-put flow's synchronous nodes no longer wait forever on a value that
is sitting in state; the consumer group drops the consumers of engines
that are gone (138 had accumulated on this installation); and the cast
that closes the long-standing `xclaim` mypy error.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/flow/test_synchronous_nodes.py

@@ -72,6 +72,37 @@ def test_synchronous_node_waits_for_all_inputs_to_be_fresh():
     assert sorted(runs) == ["eager", "sync"]
 
 
+def test_a_seeded_value_counts_as_having_arrived():
+    """A re-put flow used to come back with its synchronous nodes wedged.
+
+    Seeding wrote the value and not its version, and version 0 reads as
+    "never published" — so the node reported active and ok and never ran
+    again, with the value it was waiting for sitting right there in state.
+    """
+    runs: list[str] = []
+
+    def sync(a, b, params):
+        runs.append("sync")
+        return None
+
+    def sensor_b(params):
+        return {"b": 2.0}
+
+    b = node("b", sensor_b, provides=[spec("b")])
+    sync_node = node(
+        "sync", sync, requires=[spec("a"), spec("b")], params={"synchronous": True}
+    )
+
+    Pipeline(
+        nodes=[b, sync_node],
+        max_workers=1,
+        initial_values={"f.a": 1.0},
+    )
+
+    b.inject()
+    assert runs == ["sync"]
+
+
 def test_increment_and_multi_get():
     state = MemoryState()