Maintained simulation layers

Use maintained_layers for a custom CSV that a person or run script owns. Setup validates it but never rewrites it.

maintained_layers:
  - id: controls
    name: Optimisation controls
    data_file_name: controls.csv
    event_year: 2026
    description: Written by run_optimisation.py

A normal maintained layer has name as its first column, unique ascending contiguous year columns, finite numeric values, unique model variables, and variables that are adjustable under the model closure.

A file containing only this header is accepted as a no-op layer and produces a warning:

name

This is useful when run_optimisation.py constructs the rows before the projection. Generated and maintained layer IDs and filenames must not collide.