Biomekka 1.8.42 - Heatwave mode

Base: 1.8.41 TRICKLE 5S REASSERT.

Changes:
- Added emergency heatwave fan mode.
- Heatwave fan mode turns on at 27.0C air/water and releases only after air <= 26.5C and both water probes < 27.0C.
- Main fan is treated as circulation and ramps 60/80/100%.
- Small fan is treated as exhaust and ramps 35/60/85/100%.
- Front glass/demist fan is pushed to 90/95/100% during heatwave.
- Heatwave fan mode overrides mist wind lock and pre-storm fan behaviour because heat protection wins.
- Ceramic and back heaters are forced OFF while heatwave fan mode is active.
- Addressable LEDs and COB are blacked out from 28.0C worst air/water temp, with release at 27.5C.
- Added heatwave flags to dashboard JSON and serial status block.

Unchanged:
- Trickle schedule remains 5s ON every 3min with ON reassert.
- Goblin pin mapping unchanged.
- Storm timing/eligibility unchanged.
- Sensor addresses unchanged.