Experimental: particles are reflected at walls instead of being deleted.
- Each boundary face has a particle_policy (reflect or remove). Walls reflect by default; inlets, outlets and periodic faces remove. A BC entry may override the default, except on a periodic face.
- A reflected particle keeps its Psi, weight and ID, so the particle count and the carried scalar are conserved at walls. This changes the result of every existing case whose particles reach a wall; set particle_policy: remove to keep the old behaviour.
- search_metrics.csv adds reflections and reflection_fallbacks, and the startup banner prints each face's policy.