TSL2591 lux property can return negative values at low light — should clamp to 0

[KevinAndani]

Summary
The lux property in the CircuitPython TSL2591 driver can return small negative lux values (e.g. -3.51) in low-light/night conditions. Lux is a physical quantity that should not be negative; at minimum it should be clamped to 0. I encountered negative values while reading the sensor at night.

Repository / File
Repository: Adafruit_CircuitPython_TSL2591
File: adafruit_tsl2591.py
Context: the lux calculation computes two candidate lux values (lux1, lux2) and currently returns max(lux1, lux2) which can be negative if both candidates are negative due to noise / IR-dominant readings.

Actual behavior
lux can be negative (e.g. -3.51) under low-light conditions.

Expected behavior
lux should never be negative. When the computed lux candidates are negative (due to noise, IR-dominant conditions, or small counts), the method should return 0.0 (or otherwise clamp to a non-negative value).

Suggested fix
Clamp the returned lux to be non-negative. For example, replace:

lux1 = (channel_0 - (_TSL2591_LUX_COEFB * channel_1)) / cpl
lux2 = ((_TSL2591_LUX_COEFC * channel_0) - (_TSL2591_LUX_COEFD * channel_1)) / cpl
return max(lux1, lux2)

with:

# ensure lux is never negative
return max(0.0, lux1, lux2)

This is a minimal, backwards-compatible change that avoids reporting non-physical negative lux values.