V137 - Face-depth flux wave update (from V135)

Purpose:
- Replace the old center-depth Laplacian update with a face-depth flux update.
- This targets the artificial low-height band/reflection where bathymetry changes abruptly from deep to shallow water.
- No shallow-water propagation floor is used in this version.

Baseline:
- Based on V135 (V130 + station local radius 2).
- Does not include V136 PROPAGATION_DEPTH_FLOOR_M.
- Keeps the Quick FF patch-internal depth continuity.
- Keeps scalar source model; no Okada changes.

Wave solver change:
- Old form: accel = wave_velocity_gain(center_depth) * sum(neighbor - center)
- New form: accel = sum((neighbor - center) * wave_velocity_gain(face_depth))
- Face depth default: geometric mean sqrt(center_depth * neighbor_depth).
- Uniform-depth areas behave the same as before, because face_depth == center_depth.
- Abrupt deep-to-shallow interfaces get an intermediate coupling instead of the shallow cell alone acting like a wall.

Compile-time flags in tsunami_core.h:
- WAVE_FACE_DEPTH_FLUX_ENABLED 1
- WAVE_FACE_DEPTH_MODE_GEOMETRIC 1

If the shallow-side low-height band remains, change WAVE_FACE_DEPTH_MODE_GEOMETRIC to 0 to try the stronger arithmetic mean. If it over-transmits into shallow bays or becomes unstable, revert WAVE_FACE_DEPTH_FLUX_ENABLED to 0, or add a lower-coupling harmonic mode later.
