Phlex/FORM integration for multiple input files

[knoepfel]

This discussion is continues discussion #505, which states that the following is required for sometime after Phlex v0.3.0:

1. The framework will need to be able to update the processed data-layer hierarchy when opening another input file.
2. The framework may support a running mode where all input files handled by a data-management system are declared to adhere to a specific hierarchy, thus avoiding unnecessary checking of the hierarchy contained in each file.
3. More than one input file may need to be open at the same time to support multi-threaded execution (transitions between input files).
4. More than one input file may need to be open at the same time to support (e.g.) data-MC overlay.

[knoepfel]

Remaining hierarchy work from #395

Fixed-hierarchy registration and validation of yielded indices are implemented. The following requirements remain for multiple-file processing:

1. Update the framework’s declared hierarchy when opening an input file and propagate any changes to index routing and other affected graph structures. fixed_hierarchy::update() currently provides a local merge operation, not runtime framework integration.
2. Validate each newly established or updated hierarchy against the computational graph’s requirements, including required layers and compatible ancestry. Existing per-yield validation checks only whether an emitted index belongs to the declared hierarchy.
3. Reject incompatible input-file hierarchies with a clear diagnostic and gracefully terminate processing. Generic exception/shutdown machinery exists, but hierarchy-compatibility detection and file-transition tests remain to be implemented.

These are follow-up requirements from the agreement in #395, rather than gaps in the completed fixed-hierarchy driver interface.