V90 SHIMONOSEKI / KANMON CHOKE = KEEP 10%

What changed
- The Shimonoseki/Kanmon choke cell is explicitly implemented as a KEEP ratio, not a loss ratio.
- Grid cell (332,225) is damped so that about 10% remains through that cell.
- This means roughly 90% loss / 10% transmission, matching the intended under-resolved strait behavior.

Substep handling
- v87+ uses WAVE_INTERNAL_SUBSTEPS=2.
- Applying 0.10 in each internal substep would leave only 1% per displayed tick.
- V90 therefore applies pow(0.10, 1/WAVE_INTERNAL_SUBSTEPS) per internal substep.
- With two substeps, each internal substep keeps sqrt(0.10), and the displayed-step product is 0.10.

Base
- Starts from V89: 6%/6h global decay, stable 2x internal substeps, depth-speed exponent fix, secondary disabled, Petal removed, preset slip+depth rebuild, coast fix.
