fix: update constants precision and centralize definitions

[Copilot]

Several physical constants were specified at reduced precision and duplicated across files. GRAVITY was 32.2 (should be 32.174), SI_GRAVITY was 9.81 (should be 9.80665), and PHI was 1.486 (should be 1.4859, i.e. 0.3048^(-1/3)). Hardcoded 1.49 appeared in multiple source files instead of referencing the named constant.

Precision updates

Constant	Old	New
GRAVITY	32.2	32.174
SI_GRAVITY	9.81	9.80665
PHI	1.486	1.4859

Applied consistently to both:

• src/legacy/engine/consts.h
• src/engine/core/Constants.hpp (+ derived SQRT_GRAVITY, INV_SQRT_GRAVITY)

Centralized constant usage (refactored engine)

• Removed duplicate local PHI definitions in Transect.cpp, Runoff.hpp
• Removed duplicate local GRAVITY in HydStructures.hpp, XSection.cpp
• Replaced hardcoded 32.2 in ForceMain.cpp → constants::GRAVITY
• Replaced hardcoded 1.49 in LID.cpp → constants::PHI

All local usages now reference constants::PHI / constants::GRAVITY from the single header:

// Before (scattered across files)
static constexpr double PHI = 1.486;
g.surf_alpha[us] = 1.49 * std::sqrt(g.surf_slope[us]) / g.surf_rough[us];

// After (single source of truth in Constants.hpp)
constexpr double PHI = 1.4859;
// Usage sites reference the central constant
g.surf_alpha[us] = constants::PHI * std::sqrt(g.surf_slope[us]) / g.surf_rough[us];