Only zero is empty, and the last day is worth reading

Two separate floors were collapsing a live cartridge into a spent one, and
removing either alone changes nothing.

EMPTY_LEVEL was 5, so everything at or below 5% was assigned daysleft 0
outright. The reasoning was that a printer at 1% is out of toner as far as
anyone standing at it is concerned. But the row exists to say how long is
left, and 2% draining a point a day has two days in it. Flooring put that
cartridge beside ones that genuinely are empty with no way back to the
difference. It is 0 now: empty means empty.

int(level / rate) then truncated the division, so anything under a full day
arrived as 0 whatever the floor did - a cartridge with six hours in it was
indistinguishable from one with nothing, and the band read it as empty.
daysleft is fractional now, rounded to two places, which is about a quarter of
an hour: finer than the estimate deserves, but it costs nothing and keeps the
ordering of two nearly-spent cartridges meaningful.

daysText reads in whatever unit carries meaning: "6 hours", "1 hour", "29 min",
"4 days". Below an hour it goes to minutes with a floor of one, because
rounding hours would land back on "empty" - the same bug one rung down.

BEHAVIOUR CHANGE worth knowing: a cartridge at 1-5% with NO history used to get
daysleft 0 from the floor and land on the order list. A rate needs two
readings; with none there is nothing to divide, and the old answer was right by
accident - it said "empty" about a level nobody had watched move. It now
reports 'no history' and shows under "No estimate yet". This reaches only
printers newly added to Zabbix; anything that got to 3% the ordinary way has
the history to forecast from.

Five existing tests pinned the old rule. They recorded a real decision, so they
are rewritten to the new one rather than deleted. One of them was passing for
the wrong reason: its series (2.0, 1.7, 1.4, 1.1) drops 0.9, under
MIN_DROP_FOR_ESTIMATE, so it never had a rate at all and only passed because
the floor short-circuited ahead of the rate check. It now uses a real 20-day
drop at a tenth of a point a day and asserts the ten days its docstring always
described.

The forecast fixture's black cartridge moves from 1% to 0% so the empty band
keeps its API-level coverage, and a magenta at 3% covers hours-left end to end.

tests/test_plugins/test_supply_history.py

@@ -8,6 +8,8 @@ purchasing decision.
 
 from datetime import datetime, timedelta, timezone
 
+import pytest
+
 from plugins.printers.services.supply_history import (
     analyse, band, burn_rate, current_run, find_replacements, normalise,
     orderlist, soonest,
@@ -113,28 +115,69 @@ def test_live_level_drives_the_countdown_not_the_stored_one():
     assert result['daysleft'] == 1              # 20 / 20, not 40 / 20
 
 
-def test_a_nearly_empty_cartridge_reads_as_empty_not_as_weeks_away():
-    """1% draining a tenth of a point a day computes to ten days. It is out."""
-    result = analyse(series([2.0, 1.7, 1.4, 1.1]), currentlevel=1)
+def test_a_nearly_empty_cartridge_keeps_its_real_countdown():
+    """Only zero is empty.
+
+    This used to floor everything at or below 5% to daysleft 0, so 1% draining
+    a tenth of a point a day was reported as gone. It has ten days in it, and
+    ten days is the only thing the row exists to say. Flooring it put the
+    cartridge beside ones that genuinely are empty with no way back to the
+    difference.
+    """
+    # 3.0 down to 1.0 at a tenth of a point a day: a real, slow, measured drop.
+    slow = [round(3.0 - 0.1 * i, 2) for i in range(21)]
+    result = analyse(series(slow), currentlevel=1)
+
+    assert result['daysleft'] == pytest.approx(10, abs=0.1)
+
+
+def test_a_cartridge_with_hours_left_is_not_reported_as_empty():
+    """The other half of the floor: int() truncated the division, so anything
+    under a full day became 0 and read as empty. A cartridge with six hours in
+    it still prints."""
+    result = analyse(series([100, 75, 50, 25]), currentlevel=6)
+
+    assert 0 < result['daysleft'] < 1
+    assert round(result['daysleft'] * 24) == 6
+
+
+def test_an_actual_zero_is_still_empty():
+    """The half that must not regress. Zero means spent, and spent is ordered
+    first."""
+    result = analyse(series([2.0, 1.7, 1.4, 1.1]), currentlevel=0)
 
     assert result['daysleft'] == 0
 
 
 def test_empty_sorts_ahead_of_a_fast_healthy_cartridge():
-    empty = analyse(series([2.0, 1.7, 1.4, 1.1]), currentlevel=1)
+    empty = analyse(series([2.0, 1.7, 1.4, 1.1]), currentlevel=0)
     healthy = analyse(series([100, 80, 60, 40]))
 
     assert empty['daysleft'] < healthy['daysleft']
 
 
-def test_a_low_live_level_is_actionable_without_any_history():
+def test_an_empty_live_level_is_actionable_without_any_history():
     """A printer new to Zabbix still reports it is out of toner."""
-    result = analyse([], currentlevel=1)
+    result = analyse([], currentlevel=0)
 
     assert result['daysleft'] == 0
     assert result['reason'] is None
 
 
+def test_a_low_live_level_with_no_history_cannot_be_counted_down():
+    """A rate needs two readings. At 1% with none, there is nothing to divide.
+
+    Previously the 5% floor answered this by asserting daysleft 0, which was
+    right by accident and wrong in general - it said "empty" about a level
+    nobody had watched move. It reports no estimate and says why; the report
+    lists it under "No estimate yet" rather than on the order list.
+    """
+    result = analyse([], currentlevel=1)
+
+    assert result['daysleft'] is None
+    assert result['reason'] == 'no history'
+
+
 def test_no_history_and_a_healthy_level_still_says_no_history():
     result = analyse([], currentlevel=80)