V87 STABLE 2X INTERNAL WAVE SUBSTEPS

Base
- Starts from V86 / V85 depth-speed exponent work while preserving the prior GUI and websocket changes.

What changed
- Reverts wave_velocity_gain_from_depth() from depth/4500 back to the stable V85 value depth/18000.
- Adds WAVE_INTERNAL_SUBSTEPS = 2.
- simulate_one_step() now runs two internal numerical wave updates for each displayed model tick.

Why
- V86 made the coefficient 4x larger to get a 2x speed-up, but that also over-amplified wave height.
- V87 keeps the stable coefficient and gets the speed-up through extra internal propagation updates instead.

Decay handling
- global_height_decay_per_step() is adjusted so two internal substeps together equal the intended 15% per 6h global decay.

Expected result
- Depth-speed exponent remains approximately c ~ sqrt(depth).
- Shoaling remains h ~ depth^(-1/4).
- Global propagation should be close to 2x faster than V85 without the V86 height blow-up.
