Public C++ surface

The installed include/eon tree still ships internal headers. The Job factory has landed. Parameters stores load state and option groups in a private Impl, so sizeof(Parameters) is that pointer. Const accessors and ParametersLoadAccess are the read and write surface. Option-group types remain in the installed ParametersOptions.h. Matter still exposes Eigen members in the header (accessors return Eigen types). Treat only these as the extension contract:

Type	Pure virtuals	Notes
Potential	force(...)	Raw C arrays stay the Fortran/FFI adapter. get_ef is Eigen. hessianRange() is a defaulted virtual appended after the existing methods.
Job	run()	runFromMatter is extra, not required.
Optimizer	step, run	New methods must be defaulted virtuals, not new pures.

Capability queries (isThreadSafe, layoutFlags, supportsBatchEvaluation, hessianRange) are defaulted virtuals. Do not add another = 0 without a towncrier breaking fragment.

finiteCutoff() stays in its original slot and is deprecated. The default hessianRange() returns it, so an out-of-tree override of finiteCutoff() still colors after a recompile. Override hessianRange() for a new potential. A many-body kernel must not report its interaction cutoff there.

CI should keep grepping that those three pures still exist and that new pures are not added silently.