feat: one number per lane — the cap — and the autoscaler is the mechanism (4295)

Operator, 2026-09-23: *"auto should be always on, not a setting, so that idle
instances quiet down when not running. the number that is visible and
something the user can tweak and manage should be the cap itself the number of
running workers is handled by the autoscaling function which is always on."*

They are right, and the reason it was not built this way is worth stating: the
manual dial came first (steps 2-4) and the autoscaler came last (step 7), as
an opt-in BESIDE a control that already existed. Nothing ever asked whether
the dial should still exist once something could move it automatically. Each
step was defensible; the result was three operator settings over one number.

## `slots`, `enabled` and `autoscale` are gone

`slots` was a MEASUREMENT wearing a preference's clothes. How many workers a
lane runs is read live and moved every minute; storing it meant the operator
had to keep two numbers in agreement and the autoscaler had to be told it was
allowed to touch one of them.

`autoscale` gated the mechanism behind a choice, so a lane nobody opted in
never gave its workers back — which is why an idle instance never quieted
down.

`enabled` is derived: a cap of zero means no consumers. "Off" and "may use no
workers" were two spellings of one fact, stored separately, free to disagree.

## Two sweeps become one

`reconcile_lanes_sync` drove the pool to the stored `slots`; `autoscale_lanes_
sync` moved it away from that same number; and most of step 7's hardest
reasoning — a stored value that is a FLOOR, a target of `max(stored, current)`
— existed only to stop them fighting. Delete the stored number and the problem
is not solved, it is absent.

`size_lanes_sync` runs every minute and owns both consumers and pool size. It
also subsumes what the reconcile was for: a worker restarted at its ENV
concurrency is corrected on the next tick rather than after five.

Growth is immediate, shrink is one worker per tick. Deliberately asymmetric —
"always on" is only pleasant if the ramp keeps up, and +1/minute would take
four minutes to answer a burst. Being one worker too large for a minute costs
a sleeping process; being too small costs work not happening. For ML the
asymmetry matters most: every new slot reloads a multi-GB model, so the slow
shrink is what stops a quiet patch from paying that cost again a minute later.

## The caps ship at one, and zero for ML

Per the operator. Conservative on purpose — and a conservative default nobody
knows how to raise is just a slow product, which is the other half of what
they asked for:

    "there needs to be something that tells the user to bump those numbers to
     improve processing rate or they'd never know the controls exist."

So a lane running everything its cap allows while work piles up says so, in
its own row, with the headroom named: *"4,060 waiting and all 1 worker busy.
Raise the cap to run more at once — this machine allows up to 7."*

It fires only when raising the cap would actually help. Not when the lane is
keeping up, not when the sizing pass has room it has not taken, and not at the
machine ceiling — where "raise the cap" is advice nobody can take.

## Migration 0105 rewrites the caps rather than carrying them

The old defaults (4/2/2/1) bounded a manual control and were loose because
moving within them was the ordinary act. The number now means "the most
workers this lane may use", which is a different promise; carrying the old
figure over would quadruple the worker lane on every existing install at the
moment this deploys.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_worker_lanes.py

@@ -46,29 +46,50 @@ def test_every_routed_queue_has_a_lane_that_serves_it():
     )
 
 
-def test_defaults_are_one_of_each_with_ml_off():
-    """Operator, 2026-09-22: 'that starting value should be one of each.'
+def test_the_shipped_caps_are_one_of_each_with_ml_at_zero():
+    """Operator, 2026-09-22: *"that starting value should be one of each."*
+    Restated 2026-09-23 when the cap became the ONLY number: *"the cap
+    defaults should be 1 and 0 for the ml-worker."*
 
-    ML at zero AND disabled is milestone 422 step 6's requirement arriving
-    early: enabling the lane is what triggers the SigLIP download, and rule
-    164 allows a runtime fetch only for a feature that is optional and clearly
-    off. A default of 1 here would make every fresh install reach HuggingFace.
+    ML at zero is milestone 422 step 6's requirement: a cap of zero means no
+    consumers, and raising it is what triggers the SigLIP download. Rule 164
+    allows a runtime fetch only for a feature that is optional and clearly
+    off, so a default of 1 here would make every fresh install reach
+    HuggingFace.
     """
     by_name = wl.LANES_BY_NAME
     for name in ("worker", "scheduler", "maintenance_long"):
-        assert by_name[name].default_slots == 1
-        assert by_name[name].default_enabled is True
-    assert by_name["ml"].default_slots == 0
-    assert by_name["ml"].default_enabled is False
+        assert by_name[name].default_slots_cap == 1
+    assert by_name["ml"].default_slots_cap == 0
 
 
-def test_default_caps_leave_room_but_are_not_the_ceiling():
-    """A cap that starts at the ceiling is a rubber stamp. Each lane's cap
-    must be at least its default slots (or the CHECK constraint rejects the
-    seeded row) and must leave somewhere to grow."""
+def test_a_lane_has_exactly_one_operator_setting():
+    """The whole point of the 2026-09-23 reshape. `slots`, `enabled` and
+    `autoscale` are gone: how many workers run is a measurement the sizing
+    pass owns, and "off" is a cap of zero.
+
+    Asserted against the dataclass's own fields rather than by name, so a
+    second knob added later fails here instead of quietly reappearing in the
+    UI beside the cap — which is exactly how three settings accumulated the
+    first time."""
+    import dataclasses
+
+    settable = {
+        f.name for f in dataclasses.fields(wl.Lane) if f.name.startswith("default_")
+    }
+    assert settable == {"default_slots_cap"}, (
+        f"a lane has more than one operator default: {sorted(settable)}"
+    )
+
+
+def test_the_shipped_cap_is_never_the_ceiling_on_a_real_machine():
+    """A cap that starts at the ceiling is a rubber stamp: there is nowhere to
+    raise it to, so the nudge that tells an operator to raise it would have
+    nothing to say. One-of-each leaves room on anything bigger than a
+    single-core box."""
     for lane in wl.LANES:
-        assert lane.default_slots_cap >= lane.default_slots
-        assert lane.default_slots_cap >= 1
+        assert lane.default_slots_cap >= 0
+        assert lane.default_slots_cap <= 1
 
 
 def test_only_ml_is_memory_bound():