A toner rate needs days behind it, not just readings

Reported from the floor: two printers showing 81 and 83 percent, both forecast
to run out in a fortnight. That is a rate near 5.8 percent a day on a cartridge
barely touched.

burn_rate required four readings and a 2 point drop, and checked only that some
time had passed - not how much. Supply items are commonly polled every few
minutes, so four readings can span a quarter of an hour, and a 2 point drop
across fifteen minutes extrapolates to nearly 200 percent a day. The report then
sorted that confident wrong number into "soon", next to cartridges that really
are about to run out. It did not look broken; it looked urgent.

A rate now needs two days behind it. Two days is the smallest span that survives
a printer's daily rhythm, so one heavy morning does not become the whole picture.
Below that the answer is "not enough history yet", which the report already has
a home for: band() returns None and the row lands in the "No estimate yet"
section with its reason shown, rather than competing for attention with real
urgency.

This is the third face of the same fault. A phantom replacement truncated the
run - "2 replacements in 2.3 days" was the same printer saying so - and the rate
was then fitted to whatever short stub remained. The near-full rule and the dip
filter stop the truncation; this stops a stub from producing a number at all.

tests/test_plugins/test_supply_history.py

@@ -339,3 +339,26 @@ def test_a_real_near_empty_reading_before_a_swap_is_kept():
     points = normalise(series([40, 30, 5, 100, 95, 90]))
     assert 5.0 in [level for _, level in points]
     assert len(find_replacements(points)) == 1
+
+
+def test_minutes_of_readings_do_not_forecast_weeks():
+    """Supply items are often polled every few minutes.
+
+    Four readings a quarter of an hour apart, with a 2 point drop between the
+    ends, used to extrapolate to nearly 200 percent a day - so a cartridge
+    sitting at 82 percent was forecast to run out in a fortnight. A rate needs
+    time behind it, and without it the honest answer is no estimate.
+    """
+    minutes = 5
+    detail = analyse(series([84, 83, 83, 82], hours=minutes / 60))
+    assert detail['burnrateperday'] is None
+    assert detail['daysleft'] is None
+    assert detail['reason'] == 'not enough history yet'
+    assert band(detail['daysleft']) is None      # lands in "No estimate yet"
+
+
+def test_a_run_with_enough_days_still_estimates():
+    """The guard must not silence a genuinely slow, genuinely long run."""
+    detail = analyse(series([84, 83, 82, 81, 80, 79]))
+    assert detail['burnrateperday'] == 1.0
+    assert detail['daysleft'] == 79