V125 TRUE 15-SECOND MODEL TICK

Base
- Built from v124.

What changed
- DEFAULT_DT_SECONDS changed from 30 to 15.
- WAVE_INTERNAL_SUBSTEPS changed from 2 to 1.
- The stable v85 depth-speed coefficient remains depth / 18000.
- This keeps the effective numerical cadence similar to the old 30s tick with two internal substeps, but makes model time, rupture delay, source injection, and observation timing true 15-second increments.

Retuned time-dependent terms
- Global 6h decay remains time-based: 6% loss per 6h.
- Shimonoseki/Kanmon choke is now calibrated as 10% keep per 30s equivalent, so the 15s tick applies sqrt(0.10) per tick rather than 0.10 every 15s.

UI/runtime
- Default steps per frame changed from 2 to 4 so the visible runtime pace stays close to the old 60 model-seconds per frame.
- DEFAULT_STEPS changed from 2880 to 5760 to preserve the previous default horizon length.
