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/frontend/views/tonerForecastNames.spec.js

@@ -108,3 +108,65 @@ describe('what purchasing receives is unchanged', () => {
     expect(written[0]).not.toContain('414X Black')
   })
 })
+
+// --- how long is left ------------------------------------------------------
+//
+// daysleft used to be int(level / rate), so anything under a full day
+// truncated to 0 and the view printed "empty". A cartridge with six hours in it
+// still prints, and the row exists to say how long. Only a real zero is empty.
+
+describe('the countdown reads in whatever unit carries meaning', () => {
+  async function daysCell(daysleft) {
+    const band = daysleft === null ? 'soon' : daysleft <= 0 ? 'empty' : 'soon'
+    const base = payload()
+    base.data.data.cartridges[0].daysleft = daysleft
+    base.data.data.cartridges[0].band = band
+    printersApi.supplyForecast.mockResolvedValue(base)
+    const wrapper = mount(TonerForecast, {
+      global: { stubs: { 'router-link': { template: '<a><slot /></a>' } } },
+    })
+    await flushPromises()
+    // Only the 'empty' band starts expanded - what is already out is what the
+    // page opens on. Every band renders a header whether or not it holds
+    // anything, so the right one has to be picked by its key rather than by
+    // position; clicking the first would close 'empty' instead.
+    if (band !== 'empty') {
+      const head = wrapper.findAll('.band-head')
+        .find(b => b.find(`.band-count.${band}`).exists())
+      await head.trigger('click')
+      await flushPromises()
+    }
+    return wrapper.find('tbody .days').text()
+  }
+
+  it('says empty only for an actual zero', async () => {
+    expect(await daysCell(0)).toBe('empty')
+  })
+
+  it('reads in hours below a day', async () => {
+    expect(await daysCell(0.25)).toBe('6 hours')
+  })
+
+  it('does not pluralise a single hour', async () => {
+    expect(await daysCell(1 / 24)).toBe('1 hour')
+  })
+
+  it('reads in minutes below an hour, never as empty', async () => {
+    const text = await daysCell(0.02)
+    expect(text).toBe('29 min')
+    expect(text).not.toBe('empty')
+  })
+
+  it('never rounds a live cartridge down to empty', async () => {
+    expect(await daysCell(0.0001)).toBe('1 min')
+  })
+
+  it('reads in whole days above a day', async () => {
+    expect(await daysCell(3.7)).toBe('4 days')
+    expect(await daysCell(1.2)).toBe('1 day')
+  })
+
+  it('still shows a dash when there is no estimate', async () => {
+    expect(await daysCell(null)).toBe('-')
+  })
+})