Add species preset: Birch (Betula pendula)

[Commons-Lab-1]

Add a calibrated moisture preset for Silver Birch (Betula pendula).

Birch is a high-water species common in northern Europe and North America.
Currently covered under preset ID 1 (High water) but could benefit from
species-specific thresholds.

Steps

1. Research birch moisture requirements for saplings
2. If field data available, calibrate T_dry and T_wet values
3. Add to docs/02-species-presets.md preset table
4. Add to `firmware/treeguard-mini/src/presets.h` (if a free DIP slot exists)
5. Include region/climate zone and sources

See `docs/02-species-presets.md` > Contributing presets for format.