V86 GLOBAL SPEED 2X AFTER DEPTH RETUNE

Base
- Starts from V85.

What changed
- V85 corrected the depth exponent by changing the solver coefficient from sqrt(depth)/230 to depth/18000.
- The user observed that the resulting propagation was about 2x too slow globally.
- Because the solver coefficient behaves like c^2, doubling the actual speed requires multiplying this coefficient by 4.

Implementation
- wave_velocity_gain_from_depth() now returns depth / 4500.0 instead of depth / 18000.0.

Effect
- Keeps speed proportional to depth^(1/2).
- Keeps shoaling proportional to depth^(-1/4).
- Makes all depths about 2x faster globally compared with V85.
