Thermals: the no-reading sentinel was a positive number

Float.MIN_VALUE is the smallest POSITIVE float in Kotlin and Java (1.4e-45);
the most negative is -Float.MAX_VALUE. The overlay hides a figure by testing
`value > ARMSX2_THERMAL_NONE` against -1000.0f, so the sentinel sailed
through as a real reading and printed as "0°".

Both sides are -1000.0f now. Every use is an exact assignment or comparison
against the constant, and the plausibility gate (-20..150) means a real
reading can never collide with it.

Worth noting when this was visible: never, on a device whose zones all
resolve, because the sentinel is then never sent. It shows up only on the
devices the fallback exists for -- a readable CPU zone and no GPU zone would
print "CPU 47° GPU 0°". Found by reading the code during the ARMSX3 port,
not by testing, which is the only way it could have been found.

platforms/android/app/src/main/java/com/armsx2/Thermals.kt

@@ -22,8 +22,21 @@ import java.io.File
  */
 object Thermals {
 
-    /** No reading. Distinguished from a real 0°C, which a phone will not be at. */
-    const val NONE = Float.MIN_VALUE
+    /**
+     * No reading.
+     *
+     * MUST stay equal to ARMSX2_THERMAL_NONE in ImGuiOverlays.h -- the overlay hides a figure by
+     * testing `value > ARMSX2_THERMAL_NONE`, so a sentinel that is not below every real
+     * temperature is not a sentinel at all.
+     *
+     * This was Float.MIN_VALUE, which in Kotlin and Java is the smallest POSITIVE float
+     * (1.4e-45), not the most negative one -- that is -Float.MAX_VALUE. So it sailed through the
+     * overlay's test as a real reading and printed as "0°". The bug is invisible on any device
+     * whose zones all resolve, because the sentinel is then never sent; it shows up exactly on
+     * the devices the fallback exists for, e.g. a readable CPU zone and no GPU zone printing
+     * "CPU 47° GPU 0°". Caught in ARMSX3 review, not by testing.
+     */
+    const val NONE = -1000.0f
 
     private const val ZONES = "/sys/class/thermal"