fix(transforms): wire weather multi-input transforms

[zeusec]

Summary

This fixes the weather transform input path and the two HeatIndex correctness bugs that become active once that path is live.

• Converts AirDensity, DewPoint, and HeatIndexTemperature to tuple-input transforms instead of reading dead inputs[] arrays.
• Uses the existing tuple/Zip pattern rather than adding a new multi-input abstraction.
• Preserves the Rothfusz heat index result instead of discarding it through local variable shadowing.
• Emits "Caution" correctly from HeatIndexEffect.

Fixes #872
Fixes #870
Fixes #871

I left #899 alone as it's out of scope for this PR.

Test plan

• Format check passed on touched files.
• git diff --check
• system/test_transform_correctness PlatformIO test build passed.
• Verified on ESP32 hardware.