V85 DEPTH-SPEED / SHOALING EXPONENT RETUNE

Base
- Starts from V84.
- Keeps: secondary disabled, Petal settings removed, v83 websocket peak update flags, v82 15%/6h global decay, v81 preset height rebuild from slip+depth, v80 depth efficiency curve, v79 Shimonoseki choke and coast absorb fix, and v78 coast correction.

What changed
- The wave solver coefficient now uses velocity_gain = depth / 18000.0 instead of sqrt(depth) / 230.
- Because the solver coefficient behaves like c^2, this makes actual propagation speed scale as sqrt(depth), matching shallow-water theory c = sqrt(g*h) up to one empirical scale factor.
- In positive-depth notation: speed is proportional to h^(+1/2). Equivalently, travel-time/slowness is proportional to h^(-1/2).

Wave-height shoaling
- shoaling_factor remains proportional to depth^(-1/4), so visible wave height follows Green's-law style h^(-1/4).

Scale constant
- 18000.0 is an empirical stability/speed scale. It changes absolute speed but not the exponent.
