Define and write a serialisable RHIME run bundle

[brendan-m-murphy]

Goal

Make modern RHIME runs reproducible from saved artifacts.

Tasks

• Define required artifacts: config, run spec, canonical inputs, basis objects/operators, model spec,
  sampling settings, package versions, and output settings.
• Implement a writer for a first run-bundle format.
• Implement a reader sufficient to reproduce post-processing without reloading raw object-store data.
• Record component data needed to reconstruct derived variables.

Acceptance criteria

• A completed modern RHIME run can write a run bundle.
• A run bundle can be loaded to reproduce post-processing.
• Package versions and key provenance are recorded.