ABL precursor - sample and write time-varying inflow planes

[ia267]

Turn the periodic ABL into a precursor - sample y-z velocity planes at a streamwise location and write them, time-stamped, producing the library that successor will use.

Scope

• In a periodic ABL run, extract y-z velocity planes at a chosen streamwise location every n steps and write it time-stamped via ADIOS2 IO (see Incomapct3d append_outflow/write_outflow buffering a block of ntimesteps planes (Case-ABL.f90:1198, tools.f90)
• Define the on-disk layout/metadata (plane index <--> time)
• Build on writer_session_t. snapshot_manager does not help here, it writes strided full 3D field dumps for ParaView. There is no plane/slice selection and it's per-snapshot file model is not a time-indexed inflow database. Instead build directly on the ADIOS2 session (writer_session_t%write_data(name, buffer, shape, start, count, ..)).

Acceptance criteria

• Precursor write a time series of inflow planes at configured locations
• On-disk plane <--> time format documented and shared with successor (separate issue)