gz_waves: core - Part 1/4

[caguero]

First of a series of patches that splits the wave-simulation work. This PR lands the engine-agnostic core that every later layer builds on.

Summary

• IWaveField — the abstract wave-field interface (Elevation, Normal, ParticleVelocity, Jacobian, Update, SetParameters, Kind).
• Engine registry — an in-process token→factory map (CreateWaveSimulation, RegisterWaveEngineFactory); concrete engines register themselves from their own packages, so the core depends on none of them.
• WaveParameters — the shared input recipe.
• Wavefield ECM component (+ serialization) — the channel on the world entity carrying the recipe and the live shared_ptr<IWaveField>. Only the recipe is serialized; a replicated copy reconstructs its engine via the
  factory.
• Eval.hh / Eval.cc — the null-safe free-function query facade consumers use (falls back to still water when no engine is present). Bodies are out-of-line so
• IWaveField stays opaque to consumers; includes a new Advance() helper so a consumer never has to deref the engine directly.
• WavesSystemBase — base class for the per-engine source plugins.

The core ships only the contract — no consumers. Buoyancy and the water visual live in their own packages gz_waves_buoyancy, gz_waves_rendering), each a downstream consumer of the wave field.

[caguero]

@bsb808 , as we discussed, added WAVES_DESIGN.md and AGENTS.md.