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

**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](vscode-file://vscode-app/usr/share/code-insiders/resources/app/out/vs/code/electron-browser/workbench/workbench.html) calculation computes two candidate lux values ([lux1](vscode-file://vscode-app/usr/share/code-insiders/resources/app/out/vs/code/electron-browser/workbench/workbench.html), [lux2](vscode-file://vscode-app/usr/share/code-insiders/resources/app/out/vs/code/electron-browser/workbench/workbench.html)) and currently returns [max(lux1, lux2)](vscode-file://vscode-app/usr/share/code-insiders/resources/app/out/vs/code/electron-browser/workbench/workbench.html) 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:
```python
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:
```python
# 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.

Open adafruit_tsl2591.py and locate the lux property and its lux1/lux2 calculation. Review how the two candidates are selected, then verify that low-light or negative candidates produce a non-negative result. Done means the lux property returns 0.0 when both candidates are negative while preserving positive readings.

Written by the indexing model from the issue text.