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.

plugins/printers/services/supply_history.py

@@ -74,12 +74,16 @@ MIN_DROP_FOR_ESTIMATE = 2
 # precise-looking figure invites more trust than it has earned.
 RATE_SPREAD_FACTOR = 5
 
-# At or below this, the cartridge is done and the arithmetic stops being the
-# useful answer. A supply sitting at 1% that drains a tenth of a point a day
-# computes to ten days; a printer at 1% is out of toner as far as anyone
-# standing at it is concerned, and it is what should be ordered first. Rate is
-# still reported - only the days-left figure is floored.
-EMPTY_LEVEL = 5
+# Empty means empty. This was 5, on the reasoning that a printer at 1% is out
+# of toner as far as anyone standing at it is concerned - but flooring the
+# days-left of everything at or below 5% threw away the only number the row
+# exists to give. A cartridge at 2% draining a point a day has two days left,
+# and two days is worth knowing; reporting it as already gone puts it beside
+# cartridges that genuinely are, and there is no way back to the difference.
+#
+# Anything above zero gets its real estimate, in hours if that is what it comes
+# to. Only a reading of zero is empty.
+EMPTY_LEVEL = 0
 
 
 def _asfloat(value):
@@ -345,8 +349,8 @@ def analyse(points, rise=REPLACEMENT_RISE, currentlevel=None):
 
     rate = burn_rate(run)
 
-    # Empty is empty. Ordering by a rate below this level ranks a dead
-    # cartridge behind a healthy one that happens to be draining faster.
+    # Empty is empty, and only zero is empty. Ordering by a rate at this level
+    # ranks a dead cartridge behind a healthy one that happens to drain faster.
     if level <= EMPTY_LEVEL:
         result['daysleft'] = 0
         result['burnrateperday'] = round(rate, 2) if rate is not None else None
@@ -363,7 +367,13 @@ def analyse(points, rise=REPLACEMENT_RISE, currentlevel=None):
         return result
 
     result['burnrateperday'] = round(rate, 2)
-    result['daysleft'] = max(0, int(level / rate))
+    # Fractional, NOT int(). Truncating meant every cartridge with less than a
+    # full day left reported 0, which the bands read as empty and the view
+    # printed as "empty" - a cartridge with six hours in it was indistinguishable
+    # from one with nothing. Two decimal places is a quarter of an hour, which is
+    # finer than the estimate deserves but costs nothing and keeps the ordering
+    # of two nearly-spent cartridges meaningful.
+    result['daysleft'] = round(max(0.0, level / rate), 2)
     # Say so when the intervals disagree wildly. The number is still the best
     # estimate available; the flag stops it reading as a measurement.
     result['rateunstable'] = rate_is_unstable(run)