A quarter-tank warning that says it once, and a gauge that keeps saying it

Two halves of the same number. The warning is the moment it happened and
the colour is how things stand: it fires on the way down through a
quarter of a tank and then holds its peace, and the gauge stays red until
a base fills the lander up, which also allows the warning again.

Once, because a lander is at its most careful in the last few seconds
before it touches, and something repeating in its ear through that is not
a warning, it is a distraction.

Checked where the fuel actually moves rather than once a frame - the tank
only changes in takeFuel and payFuel, so there is nowhere else it can
cross a threshold.

Channel two, with the crash on three. Separate channels rather than one
reused, because a crash while the warning is still sounding should not cut
it off, and on a machine with four voices there is no reason to be clever.

Half and a tenth are the obvious next thresholds and are deliberately not
here: one is enough to find out whether being told at all is welcome.

---- And the crash sound is the one that was designed for it ----

Crash.json, which carries voice_gate itself, so the program's own trigger
write is gone - it would have masked a deliberate choice rather than
backing one up. The note moved from 24 to 60 by ear, and the comment
saying it was low went with it.

---- A check that could not tell a beep from a nag ----

Counting bursts of sound cannot: without the latch the warning fires
every tick the tank drops, far faster than the sound decays, so the beeps
run into each other and a burst counter sees one long burst either way.
It measures the LENGTH now - 26,944 samples against 490,348 - which is
the difference between six tenths of a second and ten seconds of it.

Tests/expected/cosmosCrossDisk.out

@@ -25,7 +25,7 @@ Mode.sbx                48
 Flip.sbx                173
 Sprite.sbx              442
 Depth.sbx               672
-Lander.sbx              8409
+Lander.sbx              8608
 Pad.sbx                 264
 Crash.sbx               632
 vars.script             50